Finally had the Siemens guy (sub-contracted from British Gas) show up to do the job last Thursday (February 22).
He actually called me at home from the place to check what needed to be done because the message he'd received from British Gas was, to use his own words, "somewhat garbled," and he wasn't sure if he was supposed to be leaving the place with a 3-phase supply or converting to single-phase.
This is the first time I've had to arrange any metering/connection change with British Gas. If I get another job which involves them, I think I'm just going to rip the seals off, make the connections myself and tell 'em to come and reseal if they ever feel like it.