I was on the 6GB, unl text, unl mins plan, paying £13.50/month with a 50% discount. I'm only about 6 months into the contract.
On live chat, I managed to 'upgrade' to the 12GB plan for £16/month. This means I get 4GB of roaming included without affecting my contract terms.
I hope this helps out a few people with sim only contracts, though I realise it's not for everyone.