could be the mechanical clock failing/failed. You may have some luck on the Internet searching for a spare. Here's one possible place you could try to contact (rather than search) http://www.alphaboilerparts.co.uk.
Looking at the manual, the closk on that boiler is an option anyway so your friendly engineer should be abole to connect a normal single channel central heating timeclock / programmer which you could mount somewhere convenient. (And maybe a room thermostat or combo clock/stat?)
There are even wireless ones available
If you've not got one, the manual is here: http://www.alpha-innovation.co.uk/custom/upload/instructions/Old%20models/HE%20CB%20Inst%20&%20Serv%20Inst%207%2005.pdf
Kelvin
November 2014
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