Thanks for the quick replies.
MM, the crashing issue is the killer for me. OK, it has only happened a couple of times despite the fact I've deliberately tried to recreate it by hitting the synth with 10+ parts and lots of polyphony, but it's left me feeling I just can't trust it. Looking round this forum and the TI Forum it seems to be quite common... shame I didn't find these forums a bit earlier.
DS, I'm more than a bit worried by your comment about having gone through 5 KC's before you got a good one. The packaging of the Polar looked OK aside from the fact that a set of staples had obviously been put in the box then removed, and a second smaller set had been used to close the box again. The synth itself was inside a heat sealed polythene bag and had a transparent film with the serial number on it covering the display and this showed no signs of having been removed at any point. The scratches on the panel are small and have obviously been spray-painted out - you can see the texture of the paint over them looks different - all of which combined made me think "reconditioned unit" or one that had the panel damaged during assembly.
Based on what's been said I'm going to try and hit Turnkey with the clause that says
"If you ordered by phone or internet and have not visited the store to discuss your purchase, you can request to cancel your purchase up to seven working days" on the grounds that I can't get it to work with my existing setup and the fact that it's got known problems with no solution at the moment. Let's hope they'll treat me decently: I haven't bought from them for a while, but they looked after me pretty well when I had problems with my last purchase, an EMU RFX-32 card.
And it looks like I'd better avoid synths with white control panels in the future.
Years back I had an even worse nightmare with an Oberheim OB-Mx - well, actually four OB-Mx's in succession because I never did get a working one....