Thanks for all your answers.
Just to share my experience...
I was paying £85 VM Ultimate volt inc a second V6 + £32 O2 unlimited data (Was £25, had 2 RPI increases during the contract).
Renegotiated O2 online a few weeks back, downgraded to 5GB, doubled to 10GB for £12/month + cost of a new phone but that was my choice so I'll ignore the cost.
Saved £20/month
Phoned VM yesterday, first offer for a like for like was £90.75/month including Netflix which I paid separately for... Typically Netflix started being offered a few weeks after my last contract started.
Managed to squeeze another £3/month out of them to get to £87.75/month and said yes
Net result, £2.75/month more to VM but saved £10.99/month to Netflix direct so £8.24/month better off + £20 better off on O2.
Yes this may be still a lot to pay for some but I'm happy with the deal, especially when the price rise notice was £21.50 a few months back, I've managed to save over £28/month
Regarding the O2 link to VM, all they asked was that this price was only being offered if you have an O2 contract, they just asked me to confirm the phone number and it went through ok.
Obviously there to make sure you haven't cancelled the O2 SIM, therefore it doesn't have to be unlimited to get a decent Ultimate Volt deal.
Hopefully I haven't bored you with my story
