How do I bathroom / kitchen / extension? etc.

Need a new boiler, can either replace like for like or could go for a condensing boiler.

Advantage would be saving on running costs, removing of airing cupboard and water tank in loft

Disadvantage - cost, plus I hear it can cause issues with old and crappy central heating (which we have)

Any thoughts/experience/words of advice?

If you've got an unpressurised feeder tank in the loft I'd look to get rid of that. Then you can decide between a combi or a system boiler + hot water cylinder. We have the later, there is something nice about getting hot water instantly to the hot taps on the same floor as the cylinder (so first floor bathrooms) and it NOT firing up the boiler to do so. But then you have a big old heavy tank.

That said having the feeder tank means you can pump to your showers which is nice if the water pressure isn't great.

Assume when you say condensing you mean combi though, because every boiler will be condensing these days.

If you go for combi be mindful that if there are very long pipe runs you might be waiting an age to get hot water through the taps.