At the thought of spending more for a battery than I did for the Plus I shaped a piece of wood to fit the battery compartment, screwed two contacts onto it made from a tin can, fastened this to a 3 x AA battery carrier and wired the contacts up.

The length of the dummy battery plus the contacts is 49.3mm.

See pics below.

http://picasaweb.google.com/bromptys/PramBatteryMod?authkey=st1AS30ZSn4

A bit redneck but it works, I will of course keep the battery compartment door.