firstly it would depend on how many hours you did in a month to work out your flight pay.then you would also need to figure out tax.
so
£10,500 per year = £875 per month
say you did 6 trips a month,each trip being 50 hours
6 x 50 = 300 hours
300 hours at £2.35 per hour = £705
so...
salary of £875 + flight pay of £705 = £1580 per month BEFORE TAX
tax deductions would then depend upon whether you are resident of the UK or spain,and the tax rate on the hourly flight pay.