Thanks for the reply Tony. Boiler selected by fitter, (who is gas safe registered), on his recommendation that though it is a 'newish' model by IDEAL it has been reliable enough for the local Mutual Housing Trust to be installing it in all their properties.
The old Jaguar had blown the PRV and there was a problem with the expansion tank. Also the diverter had begun to leak again. To top it all the gas supply pipe feeding the boiler, 1 open flame fire and a 4 burner hob and double oven was in 15mm but booklet says it should have been 22mm from the meter.
New boiler is in, running very quietly and warms up the rads a hell of a lot faster than the jaguar. Cost £1400 for new boiler, new gas supply, (approx 15-18mtrs @22mm), complete power flush using X800 Sentinel, (Shut down all rads except one, flush through until clear water flowing, shut that one down and repeat on all others followed by topping up whole system, flush it through and then add inhibitor, X100 Sentinel, and top up system). New double rad for conservatory and approx 2x7mtrs @ 15mm pipework, TRV and lockshield valve. Gas pressure was 20mb and pressure drop was 0.04mb
Does that all sound ok to you?