Back in the day I did manage 700hrs one year! Average was more like 500 though, the most I could earn was £50 p/h but that was only for Commercial courses the rest of the time it was £40 and I think that it is still representative today.
I agree with Freddie you'll always be at work, days off were when the weather was poor,but it basically had to be on the deck not to be able to something. It was hard work but I was glad of it and wanted to do the hours. If I hadn't then I'm sure I wouldn't have got into the so called 'proper job'.
I would say £25 - £30ish K would be fair for an average amount of Instructional work, to earn more you'd need to be an examiner! They rake it in!
August was always poor, everyone on holiday, Jan and Feb the weather and Skiing holidays also slowed things down a bit.