Pressure build up in RRC

Hi gang, I recently changed over my expansion tank due to a seam cracking on it and thought that was the end of my worries, Today after a little fun offroad and a drive back along the bypass i noticed for the first time since i've had the car the temp gauge just over the half way mark.
When i got home i checked under the hood and there seems to be excess pressure building up in the cooling and it forced water over the top of the expansion tank (dont ask me where it came from)
Any suggestions? Too much water in it? Thermostat? Not really too clued up on the techy side but if made easy i will give it a go. Hoping it's nothing too serious as i have a run on in may.

Help

DC

Will check that out ASAP.

Checked oil yesterday and it was fine. Nothing under the cap also, but i'll have another look and do the hand test also

Cheers

ok

UPDATE:

Cleaned all the radiator free of any mud that may have been preventing the good flow of air, Changed over my expansion tank cap as it might have been faulty and not venting at about 15psi, Today i had the gases checked in the cooling system to see if there was traces of fumes which may suggest head gasket, got the all clear, Seems to be running ok now but i am just going down to check the water level is at half way since my run out today/ Make sure it's still not losing water.

Thanks for the help, Oil was ok also nice and clean still from march' service, Oil cap clean, No smell of sweetness in the exhaust fumes that also may suggest coolant leaking into the system.

I'll keep a wee eye on it mind

DC