Tim
Unfortunately, I think you would probably be better to budget for something closer to £ 700, by the time you add on weekend supplements etc.
The flydrive basic price in the brochure is £ 730 - 11th july - 21 aug with weekend supplements @ £ 15 pp, so even with 10% disc thro frequent virgin or whatever your still left with £ 680 ish + extra weeks car hire + £ 23 per day for all inc insurance.
...... getting kinda expensive.
22nd aug - 31 aug drops to £ 629 + + + + .....
David