It really depends where you go but based on my own experience at a busy school near London in the summer I would be doing about 22 to 30 hours per week at £15 per hour + £15 per day retainer giving about £420 to £540 per week pre tax. ( I only ever got above £500 four times)
Winter is the killer though, hours can be down to zero to 10 per week giving you £90 (retainer 6 days) to £240 per week
. Total over a 12 month period was £15,800 pre tax.
Plus you might want to check out some of the other posts on Flying instructor thread regarding pay & conditions.
Good luck.