Small problem slowly developing that I wish to smack on the head soonest

After a longish run, at least 40+ miles with the 'van hitched, so the beast is working a bit harder than normal, I have noticed I've lost about a cupful of coolant - so this weekend I watched like the proverbial. The temperature never goes above 92 and is usually around 85 whilst towing - when "solo" the temp is usually around 80.
This was found on Friday night:
The leak starts about 5 minutes after switching the engine off, and is now traced to the header tank cap presure relieving.. with the header tank completely full of water ... once the system cools the water level drops to normal.
Exactly the same thing happened tonight on return ... but I deliberately started the engine and gave as couple of quick throttle blips ... water level instantly drops to normal, then slowly rises after switch off.
Conclusion .. localised heating in the system is causing local "boiling" and subsequent vapour expansion causes the level to rise and the pressure to increase enough to "crack" the expansion cap to do its job.
Now, on the passenger side at the top of the radiator appears to be an additional electric pump in the system - it's not mentioned in the haynes book-of-lies at all - is this the one for giving heating with the engine switched off as the handbook says - or something to do with cooling ??
I haven't been able to make it run yet, but will try a direct 12v across it tomorrow in the light - assuming it is working OK .. what "triggers" does it need to operate ??
Or am I heading in totaly the wrong direction

Any ideas gratefully accepted
