Didn't even consider that I'd need to sort out insurance for a private test ride..
Yes, it's a legal requirement to have 3rd party insurance on the public road.
It used to be fairly common for all policies to have a clause that you could drive other vehicles 3rd party.
However this has been quietly withdrawn so it's not universal anymore, so anyone relying on that clause needs to make sure they are covered for it.
I believe it's been withdrawn because it was open to abuse - so it was intended that you could drive your mates car if he/she was ill, not so that you can buy insurance for a corsa and then drive a Ferrari.
A case of a few spoiling it for everyone else.

If you get stuck then I'd get in touch with the manufacturer. They can't expect customers to buy their bikes if they can't test ride them.