Pressure on worcester 28 cdi keeps rising when not on. Water Inlet key not in place & rads bled.Help please

It could be the secondry heat exchanger leaking (this would be internally) which would cause your pressure to rise. That is if, as you said you had discounted the filling loop Kind regards Terry

The 2 previous answers could be correct. There is another possibility. You may have a defective secondary heat exchanger. This is the part that heats your Hot water. If it has cracked or split internally it will increase the boiler pressure even when the boiler is off. If the pressure only rises when the heating is on and it reduces when the heating has been off for a while it will definitely be the expansion vessel. Hope this helps. Fred.