Advice on possible reasons for Combi Boiler problem?

I have a Worcester Bosch 28si Junior Combi Boiler, (3 years old). No problems until very recently when having a shower at night, through mixer tap on bath, hot water running cold for periods before returning to hot. This is not the case when having shower in morning as plenty of hot water at this time.There is no problem with heating which is working fine. Wondering if not so much to do with time of demand for hot water but rather that at night the heating has been on for a few hours as opposed to just coming on when having shower in morning. When hot water on demand in evening I have checked the boiler and the pilot light indicator flashes on/off and there is an audible clicking noise which sounds different than usual noise when boiler firing up.
Any advice would be greatly appreciated.

Grant Ramsay
March 2009