We are getting closer on the tree identification, Jill Spearman sent this reply to the Dowson's.
It is some kind of citrus, but I don't know what kind. Last Autumn I picked a fruit from the tree, it was like a small satsuma but was so bitter it was inedible. It was packed with seeds which I have successfully germinated. I have now got 11 little plants about 3" high looking very healthy.
Shaun Dowson says
it looks like it could be a "Poncirus trifoliata"
this is a type of orange believe it or not.
i found it on the Internet because i thought it looks like a crown of thorns, and some people call it crown of thorns bush, or trifolate orrange.
