thank you all for your contributions - much appreciated. i CAN remove the bolt holding the block to the frame and get SOME more room to work, but it's quite limited, maybe no more than 15mm-20mm. the tube (brake line) runs from the fitting back to the ABS unit under the seat, and it's long, convoluted, and reasonably rigid. for edification, here's the schematic:
the part in question is the nut that secures the brakeline into the block at the end of hose #1. the brake line back to the ABS unit is like most brake lines, not flexible...
particular thanks to sid squid - I think a nice tight fitting 'flare nut spanner', with the block loosened up for extra working room, might do the trick. I had no idea such a tool existed, which is why I was thinking about buying a closed-end wrench and grinding a slot in the back end. brilliant. i'll report up once I have a go ( it'll be a few days till i can get to it...)