Ford escort Mark One ....bad starter?

I have an escort with a fairly newly built and rebored 711m 1600 engine bored to 1700 The car is a bad starter. I have fitted a new battery (Extra heavy duty) I have replaced the battery leads, plug leads , new distributor new solenoid. The car may only be left 1 or 2 days and the engine is very slow to turn over as though the battery is flat. Other electrics seem fine. the engine has done about 1500 miles since a full rebuild. Once the engine has started and warmed up it starts with no problem.
plugs are clean, oil is fairly new. points are set correctly I can only assume there must be a weak point in the ignition system or the battery is draining somehow.....can any one help??