Having explored various options (alu tooling plate with resistors, silicon heating mat) I've taken the easy way out and ordered a PCB Heatbed Mk1 (by Prusa) from reprapsource:
Original post on thingiverse: http://www.thingiverse.com/thing:3919
Pity about the shipping charge, but seemed to be simplest option overall and good value compared to the alternatives (~£42 inc shipping).
I also finished the RAMPS electronics last week - will get round to posting a picture soon. Next up is ordering a J Head Hotend and some parts for the Wade's Extruder from thingfarm.org.