They certainly went through our monthly income to see if we could afford repayments but I don't think it was to the same level as when you first take a mortgage out. If you've had a good payment history I think they'll be pretty flexible within reason.
And putting more on your mortgage is MUCH...
Pretty straightforward as long as you've got a decent history of paying the mortgage. They're always happy to make more money out of you.
We did it when the conversion of our house went over budget (as they always do).