got the measurements from my rack supports as promised.
288mm from outer edges of supports
230mm from inner edges of supports
so it appears that my supports are now 29mm wide instead of 30mm plate, as said i had to grind off a little on each OUTER edge to let the lugs/bracket part of the rack fittings sit flush with the CP19 CP20,
i also think it had something to do with the welds on the rack itself, so it is easier if you have the rack to aid the final placing of the supports.
hope this helps.
andy