il try and get a closer up pic of it when i get home tonight n post first thing in morning...il check to see for numbers.

that is what the Gasman said today as he turned on my immersion heater and walked out to get his lunch. however thats not working either water ran cold an hour after he turned that switch on....i literally never use it so hadn't noticed it wasn't working before ...so its the kettle for us for next few days

Cant get close up pic that shows information on the blue valve but it says this on top of the blue valva cap:

Drayton Controls Ltd. Cytrol
it also has a letter K on top the blue valva has 2 protruding nibs which appear to be on the 4/5 setting on the brass fitting

going to ring british gas to complain about being kleft with cold water now since monday evening! they have to doscount me for that at very least!

I've had this problem on my system (at least 25 years old) a few times. What I do, is shut down the sytem, connect a hose pipe to the drain valve on the heating boiler, close the vave on the circulation pump then open the drain and let it run to waste outside for a while. I then coupled the other end of the hose to the outside tap and turned it on until the moment water flows from the overflow outside the house then change back. Works every time. I think its a build up of sludge in a couple of horizontal pipes between the boiler and hotwater tank that is my problem
Trevor

thanks for all the replies.
After kicking off with british gas over being left without hot water they offered a big discount and did a powerflush 2guys worked their all day till late evening draining and cleaning the system and replacing bits along the way including a new cytrol valve system the whole system was clogged up with thick black sludge i saw it all coming out and it took a lot of flushing for it to clean out...we now have hot water and a much more functional cleaner system. long may it continue!