Loss of coolant through reservoir

[CEB]

HI all,
I have a problem with my FD3 the coolant reservoir is overflowing and thus I am loosing coolant. I have tested the thermostat and it seems to be OK.  The low level warning beeber beep at 1 second intervals temp at around 80 c
it beeps for about a minute and in that time the temp rises about 5c then tne beeping stops and the temps drops.  Eventualy the beeper is on continually for about a minute temp rise then drop. the level in the reservoir slowly becomes full and overflows.  I can take the cap off when the engin is running, verry little pressure and no water rushing out.  It i top it up slowly i get big air bubbles whn it becomes full.  engine seems fine.  It is booked into WGT on monday but not sure if I can get it there.  
Any ideas :2Confused

:cheers
Clive