Aaarg! This is a standard problem and has a standard solution. I'm referring to the ORIGINAL problem - all you others go away and start your own threads, though the solution is probably similar you're confusing the issue.
Situation is Boiler > VENT pipe > FEED pipe > PUMP > valves .
Stop using the phrase "expansion pipe" it means nothing. The expanding water goes up both the vent and feed pipes, mostly up the feed pipe, the level in the f/e tank and the vent pipe will be the same.
You have a blockage in the pipes where the feed pipe meets the system. It's probably HARD one which will not dissolve easily. You will either have to drain then cut out the pipes in the "T" area, or get jiggy with a coat hanger/curtain wire/electrician's springy rod to work what you can out of the pipes. Sometimes you can get access up the pipe which goes to the pump, but don't expect the pump valves to seal and not leak - buy new ones they're cheap, Pegler's are ok(
Screwfix).
Obviously that's messy so as a first try, get a water-capable vac. They're in the sheds for under £40. Turn the heating off and let it cool. Suck at the VENT pipe over the f/e tank (make a seal with a plastic bag wrapped round). This will take water down the feed pipe and across the blockage, up the vent pipe - sometimes it works immediately, taking bits of hard and soft goo out of the system.
Pass the vac to her below, or carry it down the loft ladder which is tricky sometimes, especially if the whole thing is full of water at 80º.
