Removing a Rega arm

Can anyone tell me the size of the nut holding a Rega rb300 tonearm to the plinth? I need to change the VTA spacers, and I don't want to have to use vice-grips.
ather than buy a wrench for the task, how about an adjustable wrench AKA Cresent Wrench that's what I always used.
Mine was finger tight and I was able to just take it off by hand. I think it's not supposed to be tightened down more than that anyway. It affects the sound quality if it's too tight.