Student finance loans - pay back straight way

What do I do?! I haven't received the letter stating how much they want yet, but I just want to be prepared incase they ask for more than I have!

You'll just have to tell them that you've spent some, can't afford to pay all of it back and ask if they'll let you set up a gradual repayment plan. Sometimes Student Finance have done this in the past - it's better to get some of the money repaid than none of it.

Unfortunately as you weren't a student, you weren't entitled to the last installment of SF and it will be asked for in full.

The last installment is fine, but the 2nd installment I did spend some of it (like I said to tide me over - maybe £700 - 900 for bills etc)
The "gradual repayment plan" does this mean like every month/week? or can I give them what I have now, then pay it back when I have over £21,000 annually?
I'm really freaking out!! I don't have the funds to pay it all back.

The loans you had whilst you were a student should still be paid back as before, once you're earning over £21,000.

The money you had when you weren't a student will probably be asked for immediately. In that case, you can explain the situation and ask if you can pay it back gradually, in installments that you can afford at intervals you can afford. There are no rules - it's at SF's discretion and the arrangement will be whatever you can negotiate with them.

Having read posts in this section of TSR for a couple of years, you aren't the first person in this situation by a long chalk. It won't be the first time that SF have dealt with someone in your position. It won't be any use to them to dig their heels in and stick with a demand for immediate payment so you should be able to come to a sensible and managable agreement.