As I've just phoned the helpline to let them know about this month's payrise, it occurred to me that there may be people out there who don't know this:
Anything you give to charity by Gift Aid can be deducted from your gross income for tax credit calculation purposes.
This doesn't seem to be made known very well in any of the paperwork, but if you phone and say 'Our household income is x, but we give y to charity by Gift Aid, so for tax credit purposes our income is (x-y) they're quite happy!
Because as Christians we tithe 10% of our income to church, this makes quite a difference to us.