Hi Wayne,
the best method and time to contact VM is to call 150 from a Virgin landline, or 0345 454 1111 from any other phone, but best to call at 8am when lines first open to avoid call queues.
When calling don't select billing, you should choose option 4 retentions (thinking of leaving us) They are generally based in the UK, and have a greater degree of knowledge and flexibility, along with a larger range of discounts than the front line agents. Hopefully you can negotiate a deal with them that's suitable to your pocket and needs in exchange for a new contract.
Do your homework first and price up similar deals with other providers. Also determine what you actually need from VM. Do you need the current level of speed, or couold you drop it down to a lower speed?