The Britony Flexiflue Multi Point Water Heater ?

The above is installed in our Church Hall and has for several months now been causing problems. When the hot water tap is turned off after use the pilot light cuts out and has to be re-lit. The unit was serviced in July 2009 and is only 5 years old. The service engineer replaced several parts and for a few weeks there was no problem. However, for the last two months I have had this problem on a daily basis and patience is wearing a little thin ! Any suggestions other than full replacement ?