I finally got through to an online 'agent' for chat, after 24 hours of frustrating attempts, but even given the incident report number, the upshot was that he could only tell me that someone should be looking at it.
Too right they should.
As a retired engineer, looking at the results of over 90 minutes of attempts at different ways to reset the box and upload its software, I can already advise them that the box is not fit for purpose, and needs replacement.
Reading posts from as far back as 2014, it seems that the HUAWEI DN370T equipment I have was already out of date and had been superceded. It is being updated with new software on a regular basis, with no attempt to upgrade the hardware to accommodate these new features. It is now over three years old.
This company seriously sucks. They are attempting to charge full fees for equipment that newer customers will have more up-to-date versions of. Company loyalty means nothing to these people - they are out to screw the last penny out of us. Wish I'd never joined them when they took over Tiscali.