G
There should be another valve that closes the cold water feed to the cylinder. If you close this then it will greatly reduce the head pressure from the header tank in your loft. It might still weep if your isolation valves arent seating, but at least it should only be a trickle.
Im not a plumber but dabble similarly to yourself
EDIT - hang on - this might only be for the hot water side ! ? Sorry - it worked when i had to change a tap ...... ??