Thanks for the replies.
We actually phoned Ariston to get someone they recommend to look at the existing boiler having had a "cowboy" look at it previously who caused it start leaking and the heat excheanger is on it's way out apparently (not sure if thats expensive to replace?). Thought it was worthwhile getting someone who was familiar with Ariston stuff rather than risk a cowboy again.
Boiler is repairable, but will cost about £500, plus it's probably had about 200 spend on repairs in the past and according to the bloke ariston recommended, it's probably not worth repairing.
The Ariston one we are considering is a quote from him, so hopefully he should know what he's doing.
The bloke who quoted for a Vaillant, seemed to think that loads of pipework changes would need doing, involving pulling down the toung & groove ceiling and taking some floorboards up above. The one who quoted for the Arison said thats not needed though, so think the other feller may have been talking rubbish.
Will probably go for the Ariston, although if they are considered unreliable, then we won't. Decisions decisions.
Thanks again.