Monarch can fly you out from Gatwick at either 0715 or 1700, with return flights at 1200 or 2145. Their total figure is �210.34.
If you'd consider Luton, the Monarch flights from there are cheaper. Out at 0730, returning at 1200, for a total of �189.50.
easyJet's prices from Luton are slightly dearer. Out at 0730, returning at 1550, for �213.96.
The cheapest easyJet flights from Stansted might not suit your needs because you effectively waste the first day of your holiday. Out at 1750, returning at 2220 for �197.96. If you're an early bird, however, you can retrieve that first day by taking the 0655 flight out. This costs just �10 extra per person. i.e. a total of �217.96.
As long as you can get to Gatwick really early, however, the cheapest flights of all are from there, with easyJet. Out at 0615, returning at 2205 for a total of �175.96.
Hoping this helps,
Chris
Links:
http://www.gbairways.com/
http://www.flymonarch.com/
http://www.easyjet.com/