Even if it is described as breathable & waterproof, make sure that it is removed for a time after any prolonged rain or your paint will suffer

If you will not be around to do this, or there's not space to dry the cover off the car then I would reccommend a carcoon or a framed cover that doesn't touch the car.

David aka "too few garages"

Peter Clayton who posts here has a waterproof framed cover for sale on Ebay ,he bought it to keep his RP in ,its Ebay item 132268622987

Thanks for that tip but I've already had one re. Folding ones from Jon E that I'm following up and hope to take delivery of soon. I'll report back about it, it was just over £200 and folds up and stores very easily when not in use.

Location: Ferring, West Sussex

A slight divergence from this thread but I think relevant.
I had very good quality outdoor cover for my 1929 short wheelbase saloon. It was for a Smart 2 seater and fitted almost perfectly. Shape correct even down to the 'ears' for the door mirrors. A little bit wide but length spot on.
I suspect a cover for the Smart ForFour would be pretty close for a LWB car.