I've just checked the combo spanner bought specially for this task & it's 1 1/8in (one & one eighth) AF. Not sure of the exact metric equiv. but it's appears to be between 28 & 29mm when checked on a ruler. If you have'nt done this before be warned that the initial flow of (hot) oil will gush out & hit the N/S exhaust down-pipe before the stream slows & runs downwards into your catch pan.
I wear a glove (to protect from hot oil and hot drain plug) and very carefully unscrew the plug.
When you feel you have reached the las thread, I then slowly ease the plug out, but tilting it upwards so as to control the oil flow. Lift up your catch pan to help your aim, then slowly pull the plug out further to speed up the flow. The catch pan will start getting heavy so you can do a sort of gradual "open plug whilst lowering pan" movement, until which time the flow should be sensible enough to judge the aim right and avoid your exhaust (which only ends in an oil shower)
And remember to fill your new filter with fresh oil before fitting it.
Good tips there Mr Noisy. I'll stash them away for next time.
I normally lie underneath shouting **** **** bollocks as hot oil pours out all over my arm and the grass.....