currently had headgasket changed with cambelt waterpump and new headboard

driving back home the car didn't feel as power ful as it did do through 2nd and 3rd gear, like there was a drop in the pull when you put your foot down?

when the car has stopped and you slightly give it some revs, you can hear the turbo kicking but then you hear a very faint 'hisss' after?

could this be a potential boost leak in one of the pipes?

Yep could well be a boost leak or possibly manifold gasket loose or something.