Yes there is a part in the middle that lifts out for cleaning.Most good plumbing shops will have one in stock or will order one for you. That is the full trap But its not expensive .
A small diy shop in my local area can supply any door colour and size 2 week turn over . So b and q should sort out your problem get on to them and dont take no for an answer.
Each rad has a in and out to and from the boiler one of your rads may have 2 ins and the other 2 outs so you dont get circulation .you will have to get someone to trace the pipework back to the heating mains.This said iam assuming your system is a 2 pipe system.Is it micro bore ?
If it was only happening on 2 rads you could have 2 returns going to one rad and 2 flows going to the other .Instead of a flow and return going to each .Thats the reason i asked.
Fitted 7 grohe bar mixers in large house we priced them up at £222 each the customer was danish. She had her sister buy them from denmark and paid 80 euros each for same mixer .Rip off britian or what .Are you going on holiday any time soon?
Think of it as a new car left in garage for a few years . Gas valve for one will more than likely be stuck .Good time to fit it as lots of fitters have v little work at present so you should get a good price