In reply to ADJC:
I've used SSS for a long time with great success, including Skye midges, and for much longer than 30 minutes. It was very popular recently up in the Lakes at an evening do when the buggers flew in and seemed to work for those who borrowed it.
1. Its cheap (£2 - £3 a bottle for 150ml so you can afford to bladder it on)
2. It doesn't destroy nylon like DEET
3. DEET doesn't smell as nice as SSS (!)