I put the camera repair off until I had saved up some pennies... Skears in Northampton have confirmed that it needs a new shutter. £260 + VAT = £312 with an estimated time of 2-3 weeks. They've kindly offered to loan me a camera for the duration as well.
Just to update/ close this thread...
I picked up my repaired camera from Skears in Northampton on Saturday. All fixed, working perfectly, happy customer!
Interestingly, the shutter count wasn't the 70,000 that I was told when I bought it; the count was a mere 24,000! Looks as though I was either unlucky or Canon used to make some poor quality shutters!
Les - Thanks for the post. I too used to crop out the lighter section, probably could have got away with it for a bit longer but got bored of the cropping/ in my head I believed I'd take a perfect shot that would be ruined by the issue!