Stuff it. Wasn't using it much anyway. I've gone full Sipgate now. It's just another thing I'm paying for that I don't need and doesn't work anyway, now.
And yes, the call centres were Closed. Sat there for about ten minutes pressing buttons until it said "we're now transferring you to an operator", followed immediately with "The call centres are closed".
Thanks, but I thought I'd try the web site one more time before contacting you and it seems to be magically working again this morning. The line test got all the way to 100% (before that it was always aborting at about 20% and saying it couldn't find a problem) and then the option to book an engineer appeared. Thought I'd better plug a phone in and check again before proceeding, and would you believe it I have dial tone again!
Spooky. In my thirty-eight years as a telecom engineer it's not my experience that this particular fault often fixes itself, but stranger things, and all that...
Unfortunately I've decided that good old-fashioned copper isn't reliable enough, so my phone will remain plugged into my Sipgate adapter. If it was cost-effective to cancel my landline I would do so but the way the Mates Rates package is structured, any such change would in fact double my monthly subscription.