my vaillant Sine 18 combi boiler has fault?

While I appreciate that it's knocking on a bit, like myself, I feel that there is plenty of life left in the old dog yet,the boiler I mean.
I would appreciate any suggestions, access to repair manual etc.
The problem: Boiler fires up and runs well for mabye a few days, then cuts out - no heat, no hot tap water.
Sometimes it will do this a couple of times a day. In short it's an intermittent fault.
A plumber friend of mine looked at it about 18 months ago ,when we had the same problem. He cleaned a few "small pipes" but said he was'nt sure if that would do anygood, however it ran without fault till now.
Yes you've guessed it, he's moved.
I,m not a complete idiot, and know my limitations and also have a great respect for gas and lecky.
I would be grateful for any suggestions, especially if they relate to the boiler problem, hmmm.
Regards
Bill.