Hi! After posting this question i put pieces of toilet paper round each of the pipes in my house and found out that the water was VERY SLIGHTLY leaking from the pipe joining onto the TRV. I tightened up this nut, it only moved a tiny amount, but nonetheless it moved, and hey presto, no more dropping pressure. I've added a couple of nails to some radiator water, but so far no sign of rust! So thanks for your advice and no, I don't need to add more protector!!!! Cheers!
Pam
June 2009