Over flowing expansion tank

hello and thank you in advance for any advice given.

My girlfriend has an old central heating/hot water system. It consists of an old boiler, a hot water tank and feed and expansion tank. A few weeks back, her hot water packed up. Then suddenly there was a loud bang somewhere in the system and the hot water began working again. But now the expansion tank has begun to overflow when the boiler is turned on. I have replaced the ball valve in the f&e tank but I think the water is coming from the vent pipe. Ive read that its possibly a split coil. Sorry for sounding thick, but is this coil in the hot water cylinder - and why would it cause the expansion to overflow? Also, are these expensive/difficult to repair?

thank you