quick search on google found one way taxi costs ranging from £45-£60 (to MAN city centre) - so you paid about the right fare - BUT - if agreed £40, then thats what you should have paid, as you say, the cab firm had your flight number, so knew your arrival time.
£3.00 airport parking should have been included in the fare quoted (as it is a "known extra") and you can no longer just wait outside, so the cabbie was aware he would have to pay for the "up to an hour" parking.
Sounds to me as though you paid the fare price, and the original quote was deliberatly low to secure the booking in the first place.