Is yours off enough that you are getting bad negatives as a result? I had a Mamiya 6 that was off only on the vertical axis and I used it as-is for a few years without issue.
It's a delicate mechanism, and the general opinion is if you're nervous about it or not really a "fix-it" type to not mess with it. Repair for them is getting difficult in the US and I have no idea what service/repair for these would be like in the UK. If you are careful, I see no reason why you can't adjust it yourself.
The rangefinder adjustment screw is hiding behind that small round cap on the upper rear of the camera body. about 20mm right of the viewfinder.