Thanks I'll give that a try when I get a second spanner big enough.
By "bite up" I assume you mean tighten? I thought that if I had a spanner on each nut then both would tighten and untighten equally as the forces are opposite even if I just turn one.
Thanks for the helpful reply. I now know what "olives" and "inserts" are. Roughly anyway.
I assume when you say to tighten the nuts clockwise you mean you are looking at the valve front on. i.e. when in my picture above if I were to tighten the nut on the right I would turn it upwards as you...
Just need some advice on tightening both ends of this valve:
This valve is drip-leaking from both ends where it meets the pipe. It happened after defrosting it after the pipe partially froze overnight. I was moving the pipe a lot to try to break up ice inside and it obviously disturbed...