Hey all,
Small update if this helps.
I'm sure I measure the round axle as 30mm although I thought the smallest was a 39mm round. No markings on it that I can tell. Now, not sure how to explain this, but the axle/spring looks like its one unit, ie, its like a half length spring welded around the axle one end and bolted to the trailer the other end.
One hub is marked Carter Wesco 150mm SO. Not sure what the SO stands for. Carter Wesco still exist and I've emailed them, but some of the guys here might be able to shed more light than them if I get a response at all (you know what companies can be like these days).
Wheels are 4 stud.
Hope some of this helps!