A final update on this saga - I now have symmetric upload speed on my Gig1 package, but at a cost. Some things I have learned whilst trying to get this sorted out:
1. I don't think there is a "ring fenced team" as mentioned by Forum Team members above. I never seemed to get through to them at any rate! The general level of knowledge about Virgin Media products within support team (both chat and phone) is variable. This isn't their fault as I expect they aren't trained / updated regularly.
2. The symmetric speed add on isn't really £6/month, well not for existing customers anyway. It seems that taking this add on is a contract variation which makes your existing contract (and any favourable pricing) null and void meaning you revert to "standard" pricing for your package "with a special discount". Therefore symmetric speed actually cost me £18/month.
3. Be wary - if an agent doesn't seem to know what you're asking for the probably don't. The agent who originally handled my upgrade didn't correctly action it so I was charged for something I would never receive. I'll give her the benefit of the doubt that this was through lack of knowledge rather than misselling.
4. The actual upgrade was done whilst I was on chat to technical support. The chat lasted getting on for three hours of being forwarded to new agents, and then the agent who seemed to know what he was doing finished his shift before it was sorted. Luckily the next guy also seemed clued up. Once he had explained the contract stuff and I'd agreed to the cost he provisioned the upgrade and it took about three minutes to kick in (including a quick service outage) so in truth pretty straight forward.
The actual broadband service on full fibre XGS-PON is really good (since a few teething troubles were sorted out as we were a new rollout area) and has been solid for the last 6 months. Dealing with Virgin Media on the other hand is very frustrating and time consuming. Hopefully I won't have to do that again any time soon. With luck some other FTTP suppliers will be available in my area by the time my contract is up and there will be some choice. That's not to say I'd would sign on with Virgin again as the product itself works well, but dealing with customer service is challenging and other suppliers might do this better.
So in summary, I have symmetric speed, but I do feel like I've been mugged