Aldrich wrote:

However, I have just discovered that the "standard" 20A junction boxes that
I have bought are only rated for 3 x 1.5 cables. One solution is to use two
junction boxes, one to handle the loop-in/out and the other to handle the
local lighting circuit. However, that seems a little inelegant and costly on
boxes.

In what way "rated"? Do you mean it's hard to reliably secure four
1.5mmsq cores in the terminals provided? Or that the covers seem to have
only three exit positions? Or some other problem?

If the first - no-one's stopping you using 30A-rated JBs, and I find the
extra space in those is usually a Win for my fat fingers.

If it's cable entries you fell you're short of, most circular JBs I've
seen have extra depth available on the existing positions, by knocking
out an extra section of the entry; it's possible that the ones you have
are supplied with say 3 entries premade, but a knockout for another
entry elsewhere. The JBs I'm familar with have Cunning rotating covers
and little marked positions to support 2,3, or 4 entries used, with two
"2-entry" positions (one at 90degrees to each other, the other at 180).

If some other problem - shout.

HTH - Stefek