Affiliate Marketing Secrets

I have been part of an affiliate program for a month. I was told that I was earning money for referrals – after about a month the site said had earned £2000 commission. When I asked to withdraw it to my back account they said that due to an error on their part the figure was wrong, and I’d only really earned £5.
I’d made financial decisions based on the figure their site gave me before, so now I’m in financial difficulty. I believe that they made a mistake, rather than lied to me, but their mistake is now costing me dearly. Surely they cannot pledge to pay an amount and then retract it? They’ve removed the ‘incorrect’ entries from the site so now it shows as me having only earned £5 from a single referral. After I contacted them all they offered is £40 store credit, which considering I’ve spent a more than twice that promoting my site (and by extension their site) believing I could afford it, is a bit of a joke.
I did try looking at the citizen’s advice bureau, but their site doesn’t help, and they aren’t answering the phone. I could only make an appointment if I took a day off work to walk into their offices and then another day off for the appointment itself.
Any help would be greatly appreciated.