Hi deepfat thanks for the reply. The coolant constantly flows from the small pipe at the side of the expansion tank. I'll investigate the jiggle valve once i find out what and where it is. As for the compression test the first results were all over the place but i hadn't done one before so i did them again ( hopefully correctly this time ) they came back as 175 on 2 and 177 on the other 2. I had also done 3 chemical tests over the last week and all came back negative. This morning I removed the thermostat housing and changed the thermostat. There was a small leak from the housing so it wasn't a wasted morning. I had done a compression test on the expansion tank, over a 6 hour period the psi had dropped just under 2. Hopefully the small leak explains that.