Thanks for the info guys. I have now done the job (and a few more besides!) and while it wasn't difficult it was awkward.
I waited until the fuel tank was almost empty then removed the exhaust and heat shield, dropped the tank so that it sat on the prop shaft. This gave me enough play in the tank to be able to wedge it on the left side of the car so that there was a gap almost big enough to get my hand up over the tank.
The lines were corroded right where they join to the hoses so for the right side it was easy as it is in two sections which join just behind the fuel tank so I had that one out in a few minutes. The left side was one complete length from the hose to the brake distributor thingy under the bonnet. I decided to remove it in two pieces and cut it just in front of the fuel tank. It then came out quite easily and almost in tact.
I took all three pieces to my local auto spares shop and they made the lines up for me while I waited.
When I got back home I straightened out the new lines (the long one was coiled up) and set about putting all the bends into the lines using the originals as a template. That was pretty easy. The harder part was to get the long one installed without deforming it too much. It was obvious that I would have to bend it around a little to twist it here and there to get it up over the tank but it wasn't too hard to do.
I tidied up the lines so they were nice and straight then clipped them into their retaining clips and connected up each end. I bought a pressurised brake bleeding kit which made the brake bleeding process a breeze - I highly recommend it!
With that done (and with me feeling proud as punch that I had done some real work on my BMW instead of just the usual filter changes etc.) I moved on to fix some other issues that had caused it to fail it's annual inspection:
1. Front Wishbones and Bushes
2. Steering Rack Gaiter
3. RTA Bushes
4. RTA Ball Joint (Where it joins with the upper control arm)
5. 2 New Rear Coil Springs
6. Handbrake Not Working on Right Side
7. Headlight Alignment
8. License Plate Bulb Replaced
9. Screen Washer Jet Nozzle out of alignment!!!!
When it actually passed it's inspection I was amazed but I was even more surprised that I had managed to do all the work myself. Thanks to this site and my trusty Haynes manual I set out to do the work and the outcome was great - I didn't rush anything, I just thought it all through logically. In total the garage had quoted about £1250 and I did the whole lot for about £480 including buying a few tools that I didn't already have.
Thanks again for all the help.