Phantom Boiler Operation at Night!?

I have a Potterton Profile 80E Boiler servicing my hot water and central heating.
My timer system is Honeywell.
I have had the system for about 12 years and by and large it has been fine.
However just recently I have noticed that after 10.30 at night, despite all the timer controls showing "OFF" as they should be, the boiler keeps firing up every 15/30 minutes or so. This continues through the night.
The radiators are not getting hot when this happens so I assume it is to do with the Hot Water side not the central heating ??!!
Somebody has suggested that it could be "Frost Stat" working but this happens every night now - not just when it is frosty or very cold.
Anybody got any ideas/suggestions what it could be and how I can fix?