I think we need to put that £20 into perspective. Jack up car, remove wheel, find leak,remove tyre,fit plug(50p) after prepping tyre,refit tyre,balance,refit wheel. That could easily be 30mins so with the cost of equipment labour overheads etc £20 isn't that excessive. Supply and demand, if you turn up with a punture you need it fixed and it's unlikley you would shop around to save £5-10.