A dealer is retaining the V5C of my ex-demo car - is this right?

I purchased a car from a main dealer who has retained the V5C for two months because it is a demo car. The dealer is also paying the tax during this period. I have read on another motoring website that...

What checks do I need to make on a pre-reg car that's been sat around?

I am purchasing a BMW 340i Touring from a main dealer. It was first registered in April 2017 but only has 72 miles on the clock and has been sat round for a while. Is there anything you would recommend...