Thanks for all your advice, had a look at creation station, they do a similar model for messy crafts called create club, prices seem to vary according to location:
Wimbledon and Ealing:
After school clubs cost £10 per weekly 1 hour club
Saturday Clubs £15 per session for a weekly or monthly 2 hour club
Holiday Clubs cost £15 for the half day and £35 for the full day (including lunch)
Which makes £12 for an hour and 45 mins seem reasonable although the holiday club price maybe a bit high.
But Stevenage for example is:
After school clubs cost £6.00 per one hour session
Saturday Clubs £7.50 per weekly 90 minute session
Holiday Clubs cost £16 for a half day session
Which is almost half but the holiday club is slightly more...
I guess you have to use a bit of trial and error. Depends how appealing the club is I suppose.
We had a great club at our school recently using the Imagineering franchise but it was only an hour and cost £3 per session and there were 30 sessions in total. So I guess £4.50 or maybe £6 at a push would be ok for 1hr 45 or 2 hours.
Trouble is, I need to factor in materials as well, hire of the hall if applicable, insurances, investing in more sewing machines - need to get my head around that and what we can make each week cheaply. I am putting together a class schedule now. Any ideas or any thoughts on other things to consider very much appreciated!