[HPsauce]

Just realised that I never wrapped this one up and thanked people for their input! So thanks folks and this is what I did:
In the end (late 2019) I bought a Drayton Wiser system, not least because there was a very good discount on offer via Amazon and we had Prime (free) at the time - pre-Xmas buying spree.

Strangely the existing Drayton backplate didn't actually fit the new controller, subtle differences that I don't understand the reason for. But swapping it for the new one (in the box) was a 2-minute job as the wiring was identical.
I've actually mounted the original backplate on a small piece of wood with a mains lead and have the old controller sitting in a quiet corner powered up and ticking over as emergency backup that can be installed in minutes.

It works fine and I've programmed it so that hot water is always on when heating may be required. That also requires disabling some of the cleverer/predictive options in the management but I don't want them anyway.
It gets information on the weather forecasts and seems to interpret that reasonably well to manage the heating more effectively than our simple wall thermostat ever did.
I don't know if we actually save money, but we don't get the "overshoots" that the old system used to produce with hot radiators continuing to warm the house up long after the thermostat said it was at the target temperature. The downside is that it can sometimes seem to take longer to warm up (than it used to) as it's managing the heat input to reach a target, but then I can always easily raise the target temporarily.....

And locating the (wireless) thermostat in a sheltered corner (well away from radiators and draughts) of our lounge works so much better than in the hallway next to the kitchen door with the oven behind the wall!

There's also remote access via an iPhone App so we can turn it back up when about to return from being away - at least that facility is there but I don't seem to have had much use for it...…..