I wanted to use the '£15 off £20 spend' codes but the jacket I wanted for my son was reduced from £20 to £16, so it wouldn't let me use the code as the price was reduced already.
So. I looked up the item in my catalogue book and input the catalogue number (which is different from the website item number) for the jacket into the website and it came up full price at £20, meaning I could now use the code.
So I got the coat for £8.99 instead of the £19.99 I would have had to pay if I ordered using the sale price.
I do hope this tip helps someone as I have encountered this problem a few times before but only just had the brainwave LOL!
I have a catalogue here so if anyone wants to do this but hasn't got one then give me the link to the item on the website and I will try and find the catalogue number for it for you!