T
tamz
is your cold water inlet the first pipe on the flow before the pump? the second one being 22mm and going over the cistern?
Probably is Fuzzy, in which case the neutral point being the feed will mean it will draw air.
Easiest fix is change it to a feed expansion.
Whoever fitted this lot has never even slept with a plumber but is about the usual quality for landlord accommodation. Surprised at the Worcs boiler tho. That is a first. Usually a Ravenheap