I did eventually use a hacksaw, but I still had to get the shop to drill half of it out.
I've completed the conversion/re-vamp of my bike to 9 speed this week, so I thought I'd post a couple of pictures of the finished work. I eventually went for 44-32-22 up front and 11-32 at the rear. It's only been a few days but I really like having a much more even range through the gears whereas as a 7 speed it had 2 big gaps for the lowest cogs. My top gear is only fractionally below my previous top one so it's effectively the same on the rare occasion I need a gear that high.
Thank you to everyone who provided advice in this thread, and also in a couple of others relating to frame rust and precisely what front wheel to get:http://imgur.com/I0sosaA
This bike's had an awful lot replaced in the last year and I'm treating it as a sort-of-new bike. In the last year it's had new:
front & rear wheels,
front & rear racks,
entire drivetrain except for the shifters - they're currently in friction mode only, but I will probably go for slightly wider handlebars in the next year so I'll keep an eye on sales and consider picking up Dura Ace 9 speed bar ends to give me the option for index shifting.
Overall I've spent about £500 on parts and work and it'll hopefully keep me from looking at brand new bikes for a few months at least