We once had a loft tank with a ball valve and there came a time when changing the rubber washer did not fix the problem entirely because the seat of the valve it sealed against deteriorated with age too. We changed the valve and all was well but we eventually opted to replace the system with a new one and a combi boiler. Given what you have said about access and maybe your health and age previously Steve it might be worth considering getting someone in to do the job for you. I find myself doing this more often as time goes by because where the mind is still willing the body often protests quite strongly - and for quite a while after when I get it wrong.
