You have to play with the figures, thus it isn't simple to do a Net to Gross calculation.
As I said earlier, really need to know the hours involved, as it is better to calculate on an annual pay basis.
Lets say you work 10 hours per day, 5 days per week, all year.
£7.50 net per hour = £19,500 net per year
£10 gross per hour = £26,000 gross per year
Using ListenToTaxman (based on typical single person's tax code)
£25,815.07 gross is £19,500 net
£26,000 gross is £19,627.60 net
Therefore, given those hours/days, £10 gross per hour gives you more take home pay, than £7.50 net per hour. Difference of £127.60
Why are you asking the question, and from who's perspective?