I'v got an 824E with SAME problem - boiler goes to lock -out (red light on ) fault code F28. Manual spouts on about gas related problem. HOWEVER the problem ONLY occurs when in HOT WATER MODE ONLY. no problem in water & heating mode. Personally (and having spoken to a few boiler engineers too) conclusion seems to point towards either a faulty DIVERTER valve OR fault on printed Circuit Board.
Trouble is , diverter valve costs around £100 and the board around £200 !!!
Mark
July 2009