ravenheat csi 85t - fluctuating water pressure?

i have a Ravenheat CSI 85t - 2005 model. Just had a new fan put in, it ran fine for about a week, then started to cut out, only get heating and occasional hot water. Checked water pressure guage and it was low - 1 bar. Topped it up to 1.4 and all ok, then after a few hours noticed the guage up to 3.8!!!

Turned it off and when cool it was at 1 bar. Looks like it may be a leak somewhere or is it dumping at pressure?

Is this going to be one of those realy long and expensive things to cure?