Apparently it is an integral filling valve. Sorry for delay in getting back to you, but I had no idea and had to ask my nephew who serviced the boiler
Thanks, XXXXX XXXXX too far away, so will have to find a local plumber, but will quote your reply. Finally, is it true that I'm doing harm to something I think was called an inhibiter that would have a bad effect on my radiators? You'll probably guess from that question that I don't have a clue about plumbing. Is there any other possibility for my problem?
If you could reply that would be great then I can finish.