Your expansion vessel (big red thing behind boiler)needs recharging. Get car tyre pump, locate valve on top of expansion vessel and connect. Manually open PRV (lift operating lever)to depressurise system. Pump up till air pressure is 10 or so psi, you will have to let out several litres of water via the PRV as you do this. It will take longer than you expect. Then repressurise system. No more hot/cold pressure variations. Try googling.
c, October 2009