On Tue, 01 Jul 2014 19:58:06 +0100, Judith in England wrote:

The grass at the side of the beds has become very, very wet - and
really never dries out: it is particularly noticeable in the morning
after the night watering session. The raised beds drain very, very
well.

I think i'd turn down the watering and keep an eye on the plants in
the raised beds.

Is aggregate the correct thing to fill the trench with - or do I use
course gravel - or something else?

Take a look at http://www.pavingexpert.com/drain03.htm and possibly
other parts of that site.

--
Cheers
Dave.