Halstead Ace Circa 2001 - Not Heating?

Problem – Intermittent fault or faults causing the boiler to cease heating water and or Central Heating.
Occasionally appears to occur when the heating is on but after Hot Water is called the boiler will ‘Lock Out’. At other time the heating of water to heating will simply cease without ‘Lock Out’ being triggered. On occasions the hot water will provide sufficient heating for a full shower whilst at other times it will only heat water for about 5 minutes when it will cease to operate. It has been known to supply sufficient hot water for two consecutive showers but then to not provide hot water for a wash/shave.
Parts changed to date [by a registered Gas Engineer] include – Diaphragm, Air Pressure Switch, Fan has been first cleaned then replaced, PCB has had dry joints rectified on two occasions (twice within last 12 months).
Following the last repair work (Air Press Sw and PCB repair) the heating worked sporadically but since being dormant for 3 days neither heat nor hot water will remain working for more than a few minutes following a Re-Set. A fluttering noise can – at times - be heard; this I believe to be a relay chattering but not sure for certain.
I need some sound advice – short of replacement - though recognise that this will one day become necessary. It would also be of great help if anyone knows of a Halstead experienced Gas Engineer in the Midlands area as I fear that this is a problem that requires detailed knowledge rather than a good general expertise.