looks like oversubscription which VM call a "high utilisation fault" (when they acknowledge it).
My suggestions are either.
1 - leave (as could be years before its fixed if even that)
2 - contact the CEO via email which will get a response from the ceo office team, this most likely will just yield you an offer to leave without penalty or ongoing discount, there is a chance they will push an upgrade through that fixes it though if lucky.
3 - make a post on VMs official forums, the staff there are more competant than the phone staff and seem more likely to recognise high utilisation faults.
The usual trick VM tried is to shift people between docsis 1 and 3 when is bad congestion but I dont know if they would put a docsis3 device on the docsis1 network plus you on the 60mbit tier which wont get put on the docsis1 network. So if you want a fast resolution and they see a fix is not forthcoming then you can downgrade your package to 20mbit or below (if they let you) and then try to get on the docsis1 network and hope thats better in your area.
BT Infinity 2 Since Dec 2012 - Estimate 65.9/20 - Attainable peak 110/36 - Current Sync 71/20