Thanks for the replies. It seems that the float level of 13.6v is the general consensus. My chargers, Dolphin, which came with the boat, do not have a specific AGM setting. The nearest one is for "sealed lead" which gives 13.6v float but only 14.2v maximum whereas the West Marine website says that AGMs should take 14.7v max. So I guess I'll go with that unless someone advises otherwise.
Cheers,
Paul