Well the conversion's finished and I'm absolutely amazed at how good it sounds. I repaired the head, put on a set of mandolin machine heads, a trapeze tailpiece,a new nut and some fairly light strings (34 22 16 12)m and tuned it like the top 4 strings of my favourite Csus tuning. I can't really describe the sound - more mellow than a tenor mandolin or a bourzouki, very resonant and a bit lute-like. It's somehow quite a melancholy sound and I can't imagine playing bright happy stuff on it, but it's fantastic for laments that have lingering notes and lots of drones. In the end it only cost about £35 including strings, so I'm well chuffed with the result.

Thanks once again for all the advice.