I had a similar problem on the same boiler, i.e. ignition going on-off resulting in a little hot water then cold, then warm, then cold, etc, and the solution was to replace the Ignition Control Box, cost about £70 plus fitting. Hope this helps
PS: I think Vokera are cr*p boilers as I get a problem with this every few months and is less than 5 years old
Shaf
November 2007