I swear there is no formula for TC, just what mood they're in that day!
Bank in a high interest account unless you are desperate, telephone them to check it is yours but TBH they always sy yes in my experience and then ask for it later.
The letters don't always come with cheques, I got one for £1K which WAS mine, but when DS got another job (he was on sick for 6 months), we weren't entitled to so much so we had to repay it IYSWIM.
If yu really need it though (and with this amount I suspect you do, we did), give them a quick call to confirm then accept it is yours for the moment: TC entitlement changes with every penny income anyhow, who knows what will happen to any of us in six months?
last year we got over £1000 from tc.we rang them to check it was right they told us to enjoy it.we asked if they would put it in writing they said no.it took a while for us to dare touch it.like i said that was a year ago.maybe that might make you feel better.
this happened to us, except we got £2500!! I got the shock of my life when I checked my balance. I had recieved the letter from the IR but not bothered to open it. I did so when id seen the money and it said in there that they hadnt paid me enough working family tax or child tax credits for this year.
My dad is a tax consultant and he had worked out they owed me £8 - so I rang him to see what he said. he told me that there is no way of knowing how they work out their calcualtions (and he is a tax consultant!) and that I should just spend it as they have obvioulsy underpaid me this year, which i thought all along.
usually he is the first person to tell me im not paying enough tax and that i need to pay the inland rev etc so if he has told me to spend it he cant be too worried that they will ask for it back....