I thought that might be the issue; I live in a flat near the centre so that doesn't surprise me. Anything I can do to flag it up so VM are aware, or is it just a case of waiting for someone to reply and confirm?
Wait and see what the forum team advise, if there's no known problem, and there's no power or noise fault, then you can raise a complaint, but the chances of that provoking action itself are low. Having said that, VM seem more likely to address over-utilisation in city centres - it is just a real pity that the company simply can't be open and honest about utilisation faults and what (if anything) they will do to fix that, including when.
I would consider a different ISP if a) they had FTTP rather than FTTC (~60Mb doesn't cut it these days, plus there's the chance they are over-utilised too),
Only a few years back 60 Mbps was seen as cutting edge, and it more than cuts the mustard for gaming, which needs 5 Mbps down and 5 up (plus a further 5 up for Twitch victims). Latency is everything, bandwidth is merely garnish unless you're a web developer or other trade who routinely download and upload terabytes. Because of differences in regulation Openreach are far less likely to suffer contention problems - they will refuse customers when there's no spare capacity, VM on the other hand are, shall we say "less thorough" in their avoidance of this problem.
If you really need both bandwidth for "heavy lift" and latency for gaming, videocalls and the like, then (should resources permit) you could consider a 35 Mbps Openreach line on a one year contract from a smaller ISP such as Zen for thirty quid a month or so, whilst keeping the VM connection for the bundle and bandwidth. Pricey, and shouldn't be necessary - is a quid a day for decent gaming a price you'd consider, and the year contract? And if it didn't deliver the latency, you can cancel in the cooling off period and pay nothing. There's a few "no minimum term" contracts offered in the market, but usually expensive for what they are, and often with companies with no solid reputation.
I'm assuming the same CMTS gear is used for the gigabit connection, so upgrading to that + the hub4 would make no difference?
100% correct. There's no preferential treatment for any data traffic, so any packets that arrive when the CMTS is running full throttle get queued. And you'd lock yourself in for a new 18 or even 24 month term at higher cost.