In all honesty rob you need to diversify your income from this site and look into stuff like non-advert annual memebrships like RTG do, you have many people that would take it up.
You're on a hiding to nothing as ad blocked usage will increase and Apple are building ad blocking and privacy controls into their OS so you can almost count on Apple users not seeing ads before long
I doubt you make £10 a user on ad revenue so giving people a £10-20 subscription option to have no ads would give you forecastable income as well as stop people seeing this junk