I had the exact same problem on my fresh built forged 4e, I was very worried when I had the call from the mot place !
dont panic, I changed my rad cap as mine was old so yours might not be holding the full pressure of the system ,
I also found my rad drain plug was slightly leaking so check that too. any loss of pressure in the system will basically allow the water to boil into the expansion. check all rad hoses and watelines. and use jubilee clips instead of the standard spring clips.
also try to run it from cold with the rad cap off for a short time to let any air release (air locked). This may solve your problem. If it persists get a sniff test done to see if its the gasket
but hopefully it doesn't come to this !
