I would let it get wet. When you get it home into a garage it will dry very quickly if you point an oscillating fan at it. Then keep it properly dry by having a dehumidifier drying out the garage.

Id spend £100 on hiring a long wheelbase Merc Sprinter van. And another £30 on two new scaffold boards. You can bring it home in the dry then.

I cover the bonnet and wings on my cars as the cat scratches the paint. However i believe they hold moisture and are really not a good idea

Hedd's suggestion is the most sensible. I assisted in transporting an Ulster in such a van to Florence and back without any problem.
If transporting on a trailer, however, I would be more concerned about whether the fabric top is sound. Any tear etc., however small, can increase at a towing speed of 50 mph, resulting in catastrophe. A warning. I've seen it happen.

Location: Melton Mowbray

David

Water is likely to enter through the windscreen, doors and side windows - but then it will do this when you use it! If you are storing in a dry garage then probably no great concern.

However, if the roof is not great, or if a fabric saloon with rotten body, you may want to offer some protection. I purchased a breathable car cover from Halfords as I was storing my saloon outdoors for a while. It's fairly soft and has two webbing straps that go under it's belly and elastic front and back so it's not going to come off whilst in transit. It's a bit saggy at the front due to not being modern car shape, but this can be fixed with a bungy cord.

It would also serve to stop anything from blowing off, the bonnet blowing open or the doors falling open.

Peter

I agree with the suggestion of a suitable van, but if that is not an option, what sort of upholstery is it? If it is leather-cloth, then I don't see a problem if it gets a bit wet - it will easily dry with no lasting damage. If it's cloth upholstery, then your idea of a frost top might be a sensible solution; I would be wary of a full tarpaulin, I have seen a nice paint finish ruined by one windy night under such a cover!

I have twice bought A7s from a long way away, a chummy and a nippy, and both times hired a van, it's much easier and less worrisome.

Location: Isle of Wight

Be very careful about using dehumidifiers around cars with wooden parts.

Wood needs a certain amount of humidity to retain its characteristics. I'm sure we've all seen wood that's dried out completely, as well as wood that's been too wet.

Thank you all for helpful comments. I have gone down the trailer route as I have another car to collect in December & will need to shuffle cars about while getting them up & running.
The garage is large & airy so lingering damp should not be an issue. I had not considered the possible fragility of the roof....it "looks ok in the photos"! We will see, but back to the wind seems best as I don't want to lose the peak over the screen! Mind you the tow car is a Punto 60, so not overloaded with horses!
I recall supplying special threaded rivet-nuts (Plusnuts!) to hold the blow moulded roof onto JCB cabs & the specification required the roof to be secure when going backwards at over 60mph. Think about it laterally... this was how they travel when not digging & the roofs were inclined to lift!
David

Location: Stratford upon Avon