How many credit cards have you got and, more importantly, what's the aggregate credit limit on them
For example you might currently have a total of £12,000 credit available to you across several cards, but only owe around £2,000. When your application for any further credit is considered (including for a mortgage) it's the £12,000 figure which is likely to be taken into account, rather than the £2,000 one. That's because you could go out and 'max' all of your credit cards the day after you got your mortgage, without needing any further approval for that additional credit.
If you've got several cards, try getting rid of most of them.
Chris