fold up rear step bolt

I'm looking for bolts to fit the rear folding step on a defender and at a glance I thought M8 but it's not. Is it 3/8 whitworth or UNF does anyone know. I tried to find the answer in a search on the site but without success.
Alan.

wheres it bolting to

It bolts into the underneath of the rear crossmember in captive nuts already there.

they should be metric but the only captive nuts are the 10 mm for towbar brackets as std

Have a look under your rear crossmember and to the left of centre you will see 2 plates (probably wrong to call them captive nuts) that are threaded tto take the bolts for the step. I've just won a step on ebay that's been used but with no bolts, I need to find some in the next few days before it turns up in the post.
Alan.