sfby wrote:
Once I've got my bags of horse manure, how do I know when it has rotted
enough to be mixed with my compost? Last year I just left it in a spare
bin for 6 months, but it didn't look (or smell!) much different to when
I started.

I'm not one of those who swear by plastic compost bins: the stuff wants
air, which is more available in a traditional heap, especially if you
turn it from time to time. If you've got a suitable place, four posts
and some chicken wire will look neat enough (to my taste, better).
Anaerobic organisms don't make compost, and do make smells. It's ready
when it no longer looks or smells like the original muck, as you seem
to realise. It's probably best to mix your muck in with other
composting material from the start.

--
Mike.