Thinking of a more structured training

A smart trainer may be a good investment here.

You can undertaken an FTP (20 minute is best) with either Zwift or Trainer Road and then the programs will use these numbers as part of the training regime(s) you choose to undertake.  I must confess though, I'm not 100% sure on how accurate a trainer's power numbers are.

The only downside is that you'll be training "blind" if you do want to incorporate any of the drills whilst you're on an actual outdoors ride and don't actually own a powermeter.

There are certain sessions that can only realistically be done with a power meter, but most can be replicated to a large degree with HRM.

What I'd say with HR zones is to aim for the middle of the targetedzone, and then let the day decide if you end up towards the top or bottom of the zone in practice. Many things can and will affect HR levels, and over time you can learn these, but if you are consistently riding at either the top or bottom of a zone, you are probably not where you should be. 

Then be aware that it can take a good two minutes for a HR to fully catch up with an effort, so don''t go out too hard in efforts. 

There are some benefits, I believe, in HR training over power. The instant effort feedback possible with power makes it so much easier to maintain an effort on the road. This means your efforts become a lot more flat. Great if you replicate that on race / goal day, but in many instances, that race / goal will not be ridden the same constant effort way as you train with power.  Therefore sometimes it is good training to ride hard over varying terrain, letting your effort rise and fall as required. 

The only issue with training in heart rate zones is that some systems use 5 zones, some 6 and some 7.  Garmin Connect which I use to plan zones has 6 zones, so when I try to do a threshold effort, it appears on Strava as a tempo effort.

Also on the road it is very hard to stay in a zone - it may be 15 beats per minute for each zone, but it doesn't take much of a hill to push me into the next zone above, or even a slight downhill means I have to pedal like a bastard to stop my HR falling out the bottom of the zone.

If you can do it on an indoor trainer you'll have better control over the weather and gradient so stand a better chance of staying in the right zone.

One thing with training is to allow enough recovery.  A lot of training programmes will specify a recovery ride, which typically is z2 for an hour maybe two hours.  I just cannot stay in that zone on real roads.

I started  more structured training the winter before last, the first year was more about an endurance build up to the 312 and endurance riding  which I sussessfuly competed and I saw some reasonable improvements in my TT's.  I used Trainer Road and a virtual power.  Last winter I did the same and saw a big improvement on my early TT's before I did by a power meter.

Nope. If you look up British Cycling and the perceived effort scale, or Heart rate training zones there is a wealth of information there.  

https://www.britishcycling.org.uk/knowledge/training-plans/article/izn20...

https://www.britishcycling.org.uk/knowledge/training/get-started/article...

If you want to get reasonably careful and structured about it do their Functional Threshold Heart Rate test, and take it from there

The advantages of a power meter are that it provides an instant response to effort, unlike heart rate which lags, and that it is not affected by temperature, tiredness, etc.   The only disadvantage is the expense.

It is certainly nice to have one (or a turbo that will measure/estimate your power output) especially for short duration intervals, but you should be able to make do without one.   I've successfully followed structured, zone-based running programmes using an HRM so don't see why cycling should be any different.  Normally you feed in your maximum measured heart rate and your training zones are calculated as a % of that (or the difference between your max and resting heart rates).   In my experience, for running, after a while you get an idea of what paces correspond to what zones - it should be the same for cycling.