It's impossible to answer as Virgin don't price by product. They start you on the price on their website and then increase it every year. The longer it has been since your landlord changed his deal the more it will be costing. It may very well work out less to upgrade to a higher tier, the quid-pro-quo is a new 12 month contract.
On the 20 MBit thing the old modems only support that as a maximum. To get the full 30 MBit you'll need a Super Hub upgrade.