Once I disconnected it, I could see that on the feed side there were
three sets of mains cable (3 live, 3neutral) the third mains used to
double socket (must be a addition to the original installation) next to
the spur.

----Aren't there 3 earths now, too? You haven't clarified this.
If none were present originally, that was a bodge.

Yep sorry - there are three earths also.

Use the size of choc-block barrel to accommodate comfortably four x 2.5mm2
conductors (at the most 30A), assuming there is enough space for 3 of
these inside the back box. Ways can be segregated to suit. Machine screws
must be tight.

I might raise the rating - just to make the fit a bit more comfortable the
15 amp was a tad tight, but I thought it to be the correct rating for a ring
main.

This is the second fitting I've found like this. The first I managed (just)
to fit directly, but it took about a half hour and a lot of swearing.
I've no idea how he (or whoever) managed to wire the spur. Its almost like
he fitted it, and then went to the other ends of the cables and pulled them
tight.