I think Maynard is talking about a share ISA rather than a cash ISA.
As such I would also suggest going the index tracker route as they're cheap and will perform averagely well: all other things being equal you'll do better than picking a random expensive fund.
Since you're looking at paying in every month you shouldn't need to worry about timing too much... however you might want to consider waiting a month or two before you start. The likelihood is that we're in for a correction, it being September and all, although past performance / future gains yadda yadda means I could be wrong.
Until I sold it I had the Fidelity Moneybuilder UK Index. Halifax also have one that might be easier to access although they have a 1% annual fee.
You're looking for zero entrance and exit fees and low or zero annual costs.
Maybe this is a good place to start looking:
http://www.fidelity.co.uk/direct/select/fu...se/tracker.html