Roger Tonkin wrote:
I'm growing a plant for the first time. Planted it in
the compost heap (last years!) and it has gone
berserk. However whilst I have 3 good sized squashes
slowly ripening, many others have seemed to set, start
swelling, then go brown and rot from the flower end.
Anyone know why this is please?

Don't know, but my courgettes are doign the same this year.

Also, how do you know when they are ripe?

I've always left them till the plants have died off completely, then
left the squash out (turning occasionally) to ripen off in the sun.
Same as pumpkins.