After a lot of indecision I decided to go with the VW insurance for the certainty that VW parts and labour will be used if its ever required - it also seemed to make a nice set with the VW service plan and warranty.
I'm planning to put winter tyres on spare wheels on the car and I asked the lady I spoke to if VW Insurance would be OK with that. She said yes, but when I pushed her and said that a lot of insurers do not like aftermarket wheels or consider them a modification she said she'd speak to the underwriter and confirm.
She called them and now I'm getting a written note confirming that their only condition is they want the rolling radius of the wheel to remain the same and they want the wheels (not tyres - wheels) installed by a garage (did not specify VW, just 'a garage'!) to confirm that the radius is the same.
I'm waiting for the written documentation to see exactly what they've noted, but I've not seen anyone mention anything like this in my insurance research online.. anyone heard of any thing like this before?