Replacement boiler advice.

I'm just looking at the possibility of replacing a 10+ year-old Potterton Suprima (non-condensing) boiler in quite a large system.

It was fitted to replace an oil boiler back in the early 00s and I don't think it was sized correctly although it never had any issue heating the system. (Would an undersized boiler run inefficiently?)
We seem to be absolutely blazing through gas! Bills have never been anything other than shockingly expensive tbh.

The house is quite big and there are three pumped zones (no motorised valves).

Two zones cover 17 radiators and the third circulates through the heat-exchange coil in the hot water cylinder.

What I was wondering is :

1) Can we install a boiler that would have more direct control over the heat-exchange coil for the hot water tank? I notice some of the Bosch boilers seem to have separate hot water controls.

2) What size boiler would typically be recommended for this in terms of kW output and what do people recommend?

The Potterton Suprima went through no less than two control panel changes and had quite a few electronic problems over the years. It's latest problem seems to be that the thermostat's not working and it's heating way too hot all the time despite being on a medium setting.

Also, do any gas boilers come with a thermostat that you can set in ºC ?
I don't like this vague 1,2,3,4,5 settings or just dots on a circle as the Potterton has.

Our old gas boiler had a nice easy to read stat!

Also, why do these boilers not have that? Seems a bit daft.

Any suggestions ?

It's a natural gas fired system with an open head tank. It was actually a sealed system when it was oil and it was retrofitted with a tank (not sure why!).

The other thing is that the system was retrofitted with a wall mounted boiler. There's a very nice chimney and flue from the oil boiler which was removed and I couldn't quite understand why a floor-mounted unit wasn't fitted instead. They seem a bit more flexible.

There's a large boiler house (part of the house) with a louvered door and a gable wall so it's currently a balanced flue system out through the wall.

I would just like to have some options in mind before I approach an installer.

We'd also like to 'tidy up' the zoning and the pumps. The plumbing's a bit of a hack job in the boiler house itself and the pipe work is far from neat and tidy. It's all 1" (I think) copper but the pumps are crammed in on top of each other at odd angles. Again, I don't know why as there's loads of space!

Are there any decent setups that would improve the controls?