Thanks guys.
Whipped of a wheel this evening and what do you know - looks like the offset is 0... being the same from the Centre hole to the piece of wood.
Sanity check:
And I did find this, "7Jx15" which I guess means 15" rim by 7" wide?
And this, which I have no clue about:
Looking at a rim on ebay as they are about $65 online.