Sensible upgrades to Trek 1.5 before thinking about new bike?

Hrm, bit of a difficult one.

First things first, change all the inner and outer cables with high-quality Shimano cables (Ultegra or Dura-Ace), this makes a noticeable difference to the feel over the brakes and gears.

In terms of upgrading the first things are wheels, tyres then groupset. Some decent lightweight wheels with good (high TPI) tyres, will cost around £300 and the 105 groupset around £300. It's unlikely you'll make much money back on old parts.

So around £600 would get you a very significant upgrade on your existing bike (make sure to factor in costs if you need your LBS to change everything for you).

To be honest, I would sell the bike and buy a new one. You can easily expect £400-500 for a Trek 1.5. So add, say £500 onto the £600 all the upgrades would cost and you have around £1000 to £1100 to spend on a new bike.

For that, you'll can expect a high-quality aluminium frame, Shimano 105 and some decent wheels (add another £200 on top and you'll be able to choose and additional upgrade, either Ultegra or a carbon frame). 

Basically, you have two choices, slowly upgrade the wheels and the groupset (paying smaller amounts at a time), or continue to ride the bike as it is for a while and save the £600 you would otherwise spend before selling the bike and buying a brand new one (which IMO is the best option).

What did you do in the end?

My Trek 1.5 triple with Tiagra is very unreliable shifting down and i have been wondering if a  105 compact 11-32 would be better and simpler. I do need that very low gear for my local hills.

Best cutting device for bike cables and outer casing is the dremel with disc cutter. Wrap steel cable in tape, gentle cut through with no bent wires or strays. Makes a very tidy cut through the outer casings

If my experience is anything to go by replacing the cables is a great first step. It's not difficult to do if you get some decent cable cutters and watch a few youtube videos. I found a big improvement in braking and gear shifting when I replaced the very basic jagwire cables on my Allez with some teflon lined/coated cables. In particular the rigid sections to go under the bar tape I think made big difference to the braking.
I also fitted some new wheels (Superstar Pave 28's), new bar tape, swiss stop pads and Vredestein Fortezza Senso Xtreme tyres. Its like a new bike and the cost was about £270

I've been running Conti 28mm tyres on the Trek 1.5 for things like Flanders and Roubaix...they fit no problem, and take some of the buzz out of the ally frame (carbon seat post also helps)

I wouldn't bother upgrading just as we're heading into winter, wait until next year/spring so your upgrades aren't subjected to salted roads/grit/cow shat/wet roads/ice/half hearted cleaning/being put away wet and dirty.
Just my $0.02

I wouldn't bother upgrading just as we're heading into winter, wait until next year/spring so your upgrades aren't subjected to salted roads/grit/cow shat/wet roads/ice/half hearted cleaning/being put away wet and dirty.
Just my $0.02

Am I the only one hearing n+1 here?

Shiny new toy and yet keeping a very nice bike that you are happy with as a winter/bad weather bike that gets upgraded parts as and when they wear out.

I have a Trek 2.1 and then I built a carbon bike this year using all 2nd hand parts.

The carbon bike flies and is much faster on hills, however I am convinced that it is the wheels rather than the frame that has made the difference.

Spend all you upgrade money on the best wheel you can afford, then you will still have them as better wheels if you upgrade the bike in a few years.

And you can keep your old wheels for winter/bad weather riding.

Not sure I'd spend £700 upgrading a bike just in case at the end of it all it didn't really feel much different. I've never done such a thing so probably not that qualified to say. If you have space for another bike then you can get a Canyon Endurance with Ultegra and their flexi seatpost for £1050 plus about £40 P&P and Rose do similar deals I think. You would still need/want a fit and I don't think there's anyway to hook up with C2W scheme with Canyon. You can maybe get similar from Planet X.

Similar to you, my first road bike was a 2013 Cube Agree GTC, reasonable carbon frame with Tiagra and Shimano R501 wheels (the cheapest ones they make, about £80 for the pair).

I ended up upgrading the wheels to RS31's because I saw a really good deal (£250 to £120 or something like that) and was feeling flush, it was a noticeable improvement over the old ones.

It didn't stop me buying a new bike with Ultegra Di2 and hydraulic disc brakes though, a purchase I've been super happy with. My best bike is far nicer to ride than the old one - I certainly could have done without but it's not a purchase I regret at all.

Going from Tiagra to Ultegra Di2 is a huge difference in terms of shifting, I'm vaguely dreading going back to the other bike for the winter I have to say - I'll have to start trimming the FD again

I think it comes down to how much cash you want to spend on your hobbies really, I get so much enjoyment from my riding that I'm very happy to spend my hard-earned on new toys - not that it's necessary, but I choose to do so.

My Trek 1.5 (9-speed Tiagra) has been up the Alpe d'Huez, Tourmalet, Aubisque, and done Paris-Roubaix and the Ronde van Flaanderen with nothing other than a pair of Fulcrum 5s running 25mm Contis as an upgrade. Might be worth upgrading to 105 just for the better braking, but I would definitely upgrade the wheels.

Have you replaced the shift cables (inc. outers)?

Nothing wrong with Tiagra, and some people have said that the hidden cables of 105 etc create more drag.

I was under the impression that c2w only applied on bikes up to £1000. Would be nice if I've got that wrong

If it's working properly I can't see why you feel the need to upgrade. Lots of people happily ride much bigger distances on similarly specced bikes as well as what might be termed 'inferior' equipment.

Perhaps the fit of your Trek isn't quite right for you. Saddle height, fore & aft, stem length and even handlebar shape can all be adjusted.

I recently looked at getting a carbon frame but after looking at the pro's and cons people put on here i decided against it and kept my bike the way it was.

As Simon E pointed out new wheels are your first point of call (make sure you get decent ones) and i'm sure after that you'll be happy with your bike as it is. The difference between 105 and Ultegra is not worth the extra cash imho.

105 or Ultegra won't be any faster and unless your rear mech has had a really hard life it should work fine if adjusted properly (and using fresh cables). A lighter bike won't necessarily 'soak up' vibration.

Decent tyres with appropriate pressures make the biggest difference to ride quality. After that some nice wheels are probably the next thing to consider.

Having said that, if you have some money burning a hole in your pocket then I'd look around for deals on 2015 models.