When water is heated it expands. To deal with this all boilers are fitted with a small expansion vessel which is like a reservoir which holds the excess water.
This expansion vessel is like a balloon in a metal pot. When the water goes in the metal pot the baloon squashes a bit to make room for the extra volume of water. When it cools the balloon pushes the water back into the heating system.
What has happened is your expansion vessel no longer works. When the water heats up there is nowhere for it to go so the pressure jumps up to 4 bar. The Worcester boiler lets high pressure water escape (most boiler allow this to go through the safety valve at say 3.5 bar).
When the water cools the pressure drops very low as the volume of water contracts.
The problem is your expansion vessel - normally a little red painted metal cylinder inside the boiler.
Hi there
when you top up,do you vent the pump inside the boiler? there is a bleed nipple on the top of it.your pressure should be around 1.5/1.7 bar when cold.lose that amount of pressure would say you have a serious leak,check around all rad pipes and look for damp patches.
hope this helps
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