Prices of not-terribly-old comparable powerchairs on eBay start at £200, which is about what I'd be paying for 6 weeks rental. I'm thinking I may need the chair for anything from 3 to 6 months. (If I'm not better after 6 months there will be TROUBLE). So I'm now bidding on eBay. When I get better I can sell it again! Or hang on to it, for any future episodes (*groan*), or things like BiCon which are physically exhausting.
This place is awesome, though: Rugged Hire. They do off-road powered wheelchairs for camping, hiking and festivals. Definitely bearing them in mind if I go to anything musical this year. No way I'd want to take my own wheelchair to Knebworth!