A few avenues worth pursuing here, especially post #10 (Condensate trap clear)
http://www.diynot.com/diy/threads/vokera-compact-he-boiler-problems.234936/
More here:
http://www.diydoctor.org.uk/forums/vokera-compact-25he-help-boiler-not-working-at-all-t38688.html
HTH, July 2016