Can anybody tell me how i use a free bet?

normally if you click to bet £5, another box will open with the option 'use free bet' .....

which bookmaker? some are unnecessarily difficult

If this is the free "compensation" £5 Sportsbook bet you are referring to that Betfair gave away this week due to the withdrawal problems, then you have to chose your bet and when the stake box appears, there is a tick box which states "use free £5 bet". You can only place within the Sportsbook however if [when] you win, the winning balance (minus the £5) then transfers and you can use on the Exchange.