A tarp is a bad idea for covering a car unless its a project in progress. Firstly, a poor-fitting plastic sheet will flap in the wind and abrade the paintwork, secondly a trap is non-breathable and will allow condensation to build up on the car, promoting rust.
Im not a fan of car covers either myself, unless its a very expensive one. Argos car covers are about as thick and effective as bin bags, so I dont know how good a tarp from them would be either. I used a cheap tarp from a diy shop to cover a compct tractor for a few weeks. Despite being very well tied and weighted down, the wind shredded it in a matter of days.
I agree with your points about them being a bad idea, but this is indeed a project, going back to a bare shell, and no space in the garage (its literally full up with the bits i have taken of the mini!)
Anyway, i brought the one in the link to argos i posted, and it fits a treat, and with a bit of rope though the eyes, tied up, i dont think its gonna go anywhere either! Bargain for a tenner