IIRC it's either 22 or 24 mm - get a cheapo set of combi spanners - Hilka or Silverline off the bay or the zone, or Aldi - the Aldi ones I bought as consumables even have a 23 mm*, so pretty useful - about £30 well spent IMO..
*=Weirdly, 23 is a size used quite a lot on Ni$$an forklifts
, so a win all round