The best thing to do is to treat it as a mistake on their part, and contact them to verify it.
I received the paperwork for the offer today. I'm a bit confused because it says the pay is £20,340, but in the phone call I was told it would be £26,000.
Then wait for their response. It's possible that they're including something else (bonuses, perhaps, or possibly free insurance or stock options) in the total value, which brings the total value of the offer up to 26k, which they had earlier lumped together and represented as the "salary". It's also possible there's been a company-wide pay cut, so their offer has been lowered accordingly. There are a number of other different possibilities as well - you can't know what's happened until you ask.
That doesn't mean that you have to accept the new offer - they made you one offer, which you accepted, but now they're showing you something different, which you're free to turn down or negotiate further.
(Side note: you say
I cannot turn down the job, which makes me wonder if the company knows or suspects that, and is attempting to lowball you into accepting less.)