You have to be very careful with joint tenancy. A lot of students think "well thats just not fair so it can't be legally enforceable". You are right its not fair but the landlord won't care. It's a way to give security for the landlord.
In reality a landlord will go for the easiest pursuable party, if one of the housemates is behind on rent and just not playing ball, if you have a joint contract he will just go for the others, or for a guarantor.
I got stung by this badly one time in a house of 4 where one guy just basically wasn't paying rent. He had all sorts of arguments that the landlord was being unreasonable, that the letting agents were complete crooks, and could argue to death why he had done nothing wrong but the basic fact was in our July to June 12 month contract he only ever paid four months: he stopped paying rent after two months claiming that there was an administrative error that meant his direct debit was stopped, which he claimed wasn't his fault, but then when the agent and landlord repeatedly asked him to pay rent by cheque or re-set up his direct debit he claimed that he couldn't because this administrative error had caused him 'cash flow problems' which was their fault. Eventually over the duration of the contract they managed to get two more months' payment from him but that still meant he was eight months short on rent. Whenever they chased him he used the same arguments - its your fault that you caused an administrative error and don't blame him that he can't pay. More than likely he had just cancelled his direct debit himself.
The rest of us paid all our rent on time right up to the end of the contract, but when it came to the deposits (equivalent to about a month's rent each) he kept all of our deposits, which covered four more months of that housemate's unpaid rent. But that still left a shortfall of four months rent. The landlord asked the other three of us to pay for it but we said forget it, you pursue him.
Then about two months later I got an angry phonecall from the Dad of one of the other housemates who was screaming blue murder at me that he had been sued for four months worth of rent when his son had paid all his rent. I explained to him that it wasn't me that had dropped short on rent, it was the dodgy housemate, but his Dad's position was basically "I don't care: sort it out between yourselves but I sure as damn am not covering because you're too childish to pay up to your responsibilities". It caused a major fall out between me and that guy's son (and we had been mates for years). In the end I talked him in to coming to a solicitor with me (the third housemate that had been paying had gone travelling round the world and so was pretty much out of contact) and we got told sorry, nothing you can do, the landlord's within his rights to get the money of either of you or the guarantor, and the solicitor said "you need to choose more carefully who your friends are in future".
In the end, annoying that it was, to preserve the friendship between me and the one who's Dad was being sued, I agreed reluctantly to pay 2 months of the dodgy housemate's unpaid rent, and he covered the other 2 months, so adding to our lost deposit both of us were down best part of £1000 because of our housemate's refusal to pay rent.