I have a HW tank and the option of using the immersion element is great ESP when the boiler breaks down (as all new ones will).
As for the amount of gas used to heat the HW tank, it's minimal as demonstrated by gas usage in summer, when CH is off.

My mind was made up the first time I went for a later morning shower after bathing the kids earlier that morning…

We have a new build with a water tank in it which presumably is mains pressure – the showers are truly awesome. Also the whole tank seems to heat up in about 15 minutes, and we have never run out of water. There's at least a bath and half of water in the tank, and who runs 6 baths an hour of hot water? Also our gas bill is about £20-£30 a month for a 3 bed townhouse. When our boiler broke one summer we didn't notice for 3 days cos there was still hot water.

If I were installing a new system tho I'd deffo go for solar, or at least a tank that supports it.

We have a new build with a water tank in it which presumably is mains pressure

No – that will be gravity fed I would assume. A combi boiler is mains pressure.

I don't think it's gravity fed. The water pressure is very high even in the upstairs with only a few feet between the shower head and the loft. I seem to remember hearing that the hot water tank is force fed at mains pressure rather than having a header tank in the loft like we used to do at my parents' house.

MF – mains pressure cylinder (unvented) or a heated thermal store will give mains pressure hot water.

Renton, if you are thinking of moving in a few years, will you get payback from spending £2000 + on a new boiler in 4 or 5 years? I would think not, despite what a lot of people would claim. The biggest way you can influence your fuel bills is how you use it. A 1 deg change on your thermostat can be a 10% fuel saving, turning it off when you got out, turning it down when you are busy, turning it off half an hour earlier when you go to bed etc.

I would give this advice to one of my customers if they were in this position, obviously can't comment on your particular installation, but there is something to be said for 'if it ain't broke, don't fix it'.

You could possibly fit a pump to boost your water pressure, radiators can be changed regardless of which boioler you use. Saying that the pipewrok that is supplying them has to be big enough to supply them with enough water. Mail me any queries if you like, think I posted my address earlier.

Oh and I would never advise one of my customers to have a combi, for some they are the only option, but if you have the room, and money go for unvented.