hi all having trouble with 7 year old halstead combi boiler. keeps loosing presure have been told it needs new expansion vessell this will cost over £100 for part and is a 2 man job as boiler has to come off wall. would it be poss/ cheaper to fit an exernal vessell below the boiler. if this is poss can any make vessell be fitted as long as it is correct size. any help would be very welcome