Originally I tried replacing the internal DC socket with an external one, and although this worked the connection was a little temperamental and would only charge if the plug was in a certain position and looked a little ugly with wires hanging out of the unit, this wasn't really that useful either.
After a bit of head scratching while thinking of the best way to proceed I suddenly thought of the mini USB connector which is used to connect the unit to a PC to transfer data to / from the SD card. Mini USB plugs are readily available and provide an excellent electrical connection once they are plugged in and USB car chargers can be found on ebay for under £3 each. Initially I intended to remove the PC functionality from the unit (as I had an SD card reader), but I found that the unit can charge the battery and still works as it should when connected to a PC even on the 500mA a PC USB port provides.
The modification is very easy, involving soldering a single wire inside the device and means I now have a fully functional GPS unit again.
Step 1: Tools / Materials
Automatic centre punch
2mm Allen Key / screwdriver
Basic soldering skills & equipment