Well thanks for all your input but I have gone for a Camelback Dart.
It's a 1.5 litre and has a pouch for the phone and associated gubbins. Test drive on a 20 miler this Sunday.
While you are here...first marathon, bit of a hilly bugger, training since June. Half marathon last week. Was going to do 20, 22, 24 miles then three week taper.