I believe that what you have to do (as we have, frequently) is book 2 weeks and then you pay the excess when you leave
Saying that, we've parked in Manchester before and they've refused to accept the extra days
Think it normally works out at around £5 a day
We'll be parking at Stansted next week and, after I'd e-mailed them, they said to just mention when we collect and they'd sort out any balance