Sorted - I finally twigged that the locking nut is concentric with the tensioner wheel..
For anyone in a similar situation, to replace the ancillary / auxillary / power steering and aircon belt on the 1.8gdi engine (in my case a 2001 model):
Slacken off the 15mm nut on the outside of the tensioner wheel that sits between the aircon compressor (beneath) and PAS pump (above). There's maybe 20mm clearance between the nut and the inner wing / chassis rail so you won't get in there with a socket or cranked ring spanner. I used the open end of the spanner.
Fully slacken off the adjustor bolt for the tensioner, which is located in the front of the tensioner housing - perpendicular to the principal axis of / at the same height as the tensioner wheel. If you look inside the housing from the centre of the engine bay you can see the block to which the tensioner wheel mounts, through which the adjustor passes. Keep winding the adjuster anti-clockwise until this block is up against the front of the housing.
The belt should now be very slack; remove it from each pulley in turn, pulling it off sideways and twisting it as necessary to get clearance with the inner wing.
Fit the new belt (which is a 5PK1343) by looping it around the tension / PAS / aircon pulleys first, then finally over the crank pulley to their rear. You'll need a longish pokey stick or two to push the belt past the crank pulley (clearance is tight with the inner wing) then another to push it over the pulley. It's fiddly but not difficult as the tensioner affords lots of slack once backed right off.
Once you're sure that the belt is correctly seated on all pulleys, re-apply tension by winding the hex-adjustor on the tensioner clockwise again - until the correct tension is reached. In the absence of a better method I stopped when I could still twist the top run of the belt by 90 degrees or a bit more with my fingers.
Finally, nip up the 15mm locking bolt on the outside of the tensioner wheel. Remove all your tools from the engine bay and run up the engine to ensure all is well.
Done
