Basic system details:
Conventional gas boiler on ground floor, fully pumped system. Cold water &
expansion tanks in loft.
42" cylinder with immersion and tank stat upstairs. This cylinder new in May
2008.
Pump, motorised valves to DHW & CH circuits.
1 room stat.
Upstairs heating on separate zone, switched manually as req'd.

Just in the last few days I am getting water coming out of the tank overflow
pipes.
This only happens when the boiler is running.
Turn off the boiler and the overflow stops in a short while.
Turn boiler on and the overflow starts again in a short while.
The system has previously worked faultlessly for some years with
annual servicing and routine repairs as required. It has not had any
modification/repair since May 08 when new HW cyl fitted.

Last night I shut everything down and on return home this evening no
apparent leakage.
Switch boiler on and within 15 mins it is overflowing again.

The pump is working and is on minimum setting, so I don't think it is
overpumping.
The motorised valves and stats seem to be working correctly.
I don't think it is a ballcock failing or the HW cylinder failing as there
has been no leakage in 24 hrs with the system turned off.
I have not yet checked the loft tanks but the leakage appears to come from
the CH expansion tank.
(crawl space only and I'm feeling my age right now)

I can't fathom what the problem might be.

Any ideas please?

Nick.