Well the tech just came out and, as is typical for this problem, all seemed reasonably OK while he was here!! He checked everything on the RF side, even put a line tester on our cable drop, checked for any corrosion on the cable, etc and couldn't find anything wrong.
Then, about 15 minutes after he left, more T4 timeouts on the upstream an dropped a channel again...
With regards to the speeds, he gave me the same line as tech support - that VM only acknowledge speedtest.net and don't pay attention to speeds of anything else. Which is nice, as all I like to do all day long is run speed tests and look at the results!
However, we also got a letter through the post today informing us of some maintenance work next week - funny how they have maintenance work scheduled when there's NOTHING WRONG WITH THE NETWORK!!! So I guess I'll see if that changes anything, and take it from there. I do wonder why no-one at tech support claimed to know anything about any ongoing or planned maintenance work when I asked them? I also wonder why one (UK based) phone tech support operative claimed they had their own internal speed test which showed my line as OK, but everyone I've spoken to since doesn't seem to know wof its existence, and/or says they just rely on customers' speed test results...