Overall wattage saving between £900 and £3000 road bike

The 80s steel bike with quill stem will have a tiny frontal area compared to the modern bike....

Fitness is a little less but only a little, Im still good for an hour physically hard cycling.  The TT is rolling with some long drags and I can see the greater drop in speed as I near the crest.  7 mile climbs in Tenerife were just slower, similar HR 

Messed that quote up good and proper somehow !

Most of your gains in speed are physical.  Being lighter and fitter will give you way more in performance than upgraded bike. 

I know weight being one of the biggest factors. Example I weigh a stone more than last year and my times for a 10 mile TT are a minute slower, same bike ,same clothing.   My time on one hour climbs are 10mins slower.  

I'm not saying dont get a nice new aero looking bike, just be aware the actual savings in terms of power are minimal for the average Joe rider.  The upside of new bike is, you will want to ride it more, get fitter and lose weight through more cycling.

They say the rider is ~80% of the drag. So the bike is around 20%.

Say you pedal at 200-250W. The bike is around 40-50W of that (less the rolling resistance on the tyres).

A more aero bike is going to be 10-20% more aero? Really does depend a lot on the bikes you are comparing. Even at 20% you're saving about 10 watts.
I believe you can save more than half that by changing to an aero handlebar setup, ie aero bar and no tape on the top/middle section.

....that said, my Canyon ultimate SLX is about 3-5 km/h ave faster than my Genesis croix de fer on the few times i commute on it.

I put this down to stiffness when accelerating from lights, through traffic, and any aero benefits come from not having mudguards as opposed to 40mm carbon hoops and fancy handlebars. 

Surely most £900 bikes these days (aluminum, with race geometry) should outperform anything any pro was riding in the 70's, 80's, and 90's. 

The placebo effect may come in to play though. Mindset that expensive bike is faster,  so you ride a little harder. Buy the bike that excites you the most!

Get the bike you like and the function it performs.  Dont expect any substantial speed benefits from upgrading a cheaper bike to expensive. The gains are minimal from bike equipment. 

You would get a much bigger jump in speed if you rode on the drops more and wore tight lyrca clothing.

https://wattshop.files.wordpress.com/2016/03/wattsavings3.png

This might give you a rough idea.

The biggest difference you can make is a set of aero wheels, closely followed by an aero frameset. The trouble is, each of these is going to set you back about £2000-3000.

Off the top of my head (from what I remember from researching bike aerodynamics at uni), about 80% of the resistance a rider feels is aerodynamic drag. Of that aerodynamic drag, about 80% is caused by the rider. So getting low on your bike (whether it costs £900 or £6000) is the most important factor in drag reduction.

In terms of the biggest drag reduction for your money (after getting lower, narrower and more aero, as this isn't always possible), comes super supple tyres with latex innertubes - potentially around 10-15 watts saved per wheel and some fitted aero race jerseys/shorts, again another 15ish watt saving.

The first priority should always be ensure the bike fits you well and that you're comfortable on it, lots of potential efficiency losses/gains there.

The 80s steel bike would look a lot cooler.

Some data from tests by highly respected Tour magazine show it's much less than the brands would have you believe (source) - typically around 1 minute over 4 hours of riding. But then again, hard facts and marketing are not comfortable bedfellows.

Specialized claim the Venge 50 secs faster than a 1980s steel bike over 40 km, using the same wheels.

If you want to be more aero then much larger (and cheaper) gains can be had by wearing a skinsuit, an aero helmet, lower your bars to get a flatter back and practice riding in the drops or crouched over the hoods. But don't be surprised if it's not terribly enjoyable.

If you're not racing it's irrelevant. Just buy the bike you like best (one that fits you well, of course).

Also, you could do a lot with that £2,100 you will have saved.

I know specialized put out these sort of stats when comparing their tarmac road bike with their venge aero road bike, but the tarmac is usually of a comparable price tbh (and obviously, comes out slower/less watts etc)

Theres probably something out there closer to what you're after