Tight Squeeze!

[Ian14]

Can anyone suggest how I can hold a service valve which is behind a plastic waste pipe ? There is no room to get in with normal grips and I can't cut the waste as there is already a joint at that point. I think I can get at the nut which needs to come off with a tap spanner but don't see hoe to hold the fitting. Someone has used two flexis to connect to a bathroom tap and there is avery small leak on the join between these two pipes. have tried sealing the leak but the cold pressure keeps the drip going.
Any advice welcome !