If you don't get any luck here, try The Red Cross, they run a medical loan service. Not sure where the nearest branch is, but they did have one in Harrow that used to do it too.
Go and see your GP for referral to wheelchair services at St. Ann's Hospital (actually it's now called Mobility & Seating Services or something equally ridiculous). They should provide you with a wheelchair for free. Ask the doctor to make it an urgent referral.
That's great thank you Pat!! so annoying that the GP doesn't actually suggest this. Will call them first thing Monday morn.