I have both of the 1st 2 games on GOG and on the ordering screen it shows I will get £6.20 store credit back when pre-ordering the game, yet when I go to the checkout it shows I will only get £5.10 back.

Which is it?

I got £6.50 ...that was a while ago last time i looked it was which was yesterday it was showing £4.10 in storecredit ... today £6.20 ... i think i was luck.

I think it's based on the USD vs GBP exchange rate so it can go up and down but not sure why your getting 2 different values. If it say £6.20 that is what you should get.

Just tried to check it it shows £5.10 in credit ... mess up i think contact support about it.

I've contacted support and see what they say.
It won't stop me from ordering, just it seems like a glitch on the system that could technically be seen as fraudulent (if you don't notice it before you click "Buy"), even if it is for a measly £1.10.

There was a glitch when I was pre-ordering TW3 in october 2014 from Russia in rubles. I was charged more than should have been, sent them screenshots and they returned the difference. I think they will refund, GOG are nice guys )