Helping everyone,
Thanks for the replies, but I couldn't find anyone to repair it....
If it helps anyone, here's what I tried:
As a last resort I tried turning the turrets and working them from one side to the other. I took the scope off the rifle and put it face down on a mirror to centralise the reticle (got this tip from somewhere online). You can just about make out two sets of crosshairs, line the two up so that they appear to sit on top of each other.
I then tapped the turrets and scope body lightly with a
screwdriver handle a few times and kept adjusting the turret extreme to extreme. I repeated this a few times and kept turning the turrets. Re centralised the turrets one last time and then remounted the scope and hey presto! Back to being able to zero in.