
right for starters the fast idle when starting a cold engine is NORMAL it's part of the autochoke mechanism
fiddling with screws is NOT recommended unless you are plugged into a gas analyser.
why dint you say all this at the start

On for about 5or 6 minutes but then she just dies.
which means your mixture is wrong at idle, as the choke comes off in that time... fiddling with the screw didn't help..as it's now too rich.. and the air leak you have is not helping
You need to get it down a garage and have them reset the mixture PROPERLY..and find the leak