Re: Car pulls to the left...
18"s do pull around a bit more, you get more tramlining and a lot more torque steer, but on a level bit of road, such as a motorway in the middle or outside lanes (Trucks wear inside lane) it should go straight, if it does not then either pressures are out, tyres are worn unevenly one side form running with wrong pressures in the past (for some time), or the tracking is bad.
Did quik-fit sell you tyres/wheels? if they did they will tell you anything to fob you off, and as other said I am not sure they could check tracking on a go-cart let alone an alfa.
Take it to a proper place, get it checked, and also check that the tyres fitted are ok all round.
Assume these are new tyres not second hand with wheels right?
Giulietta QV 1750TBi (been&gone= 156, GT, Abarth 500)
Mrs TallPaul - 159TI sportwagon 2.0JTDM