You have formatted the disk first haven't you?
Nah, actually, I didn't want to format the disk until I knew I had a recoverable image of the OS. Somebody mentioned dial-up and I realised there could be a problem with the wireless drivers (not uncommon for Apple products) so I tried it with my Ethernet cable. Success. Download time ~2 hours. Thank God for 'legacy' Ethernet ports eh.
That's only part solved the issue because Apple have somehow managed to make the Lion install image slightly larger that 4.7GB so I can't burn it to any of the DVDs I've got kicking around. I've had to order a 16GB USB stick. At least I'm making progress.