I have downloaded (and purchased) mail on Sunday royal wedding, but it will not complete installation. Any suggest suggestions?

I have just tried the preview on my iPad running the same iOS as you, it will not install, I could'nt bring myself to buy it even at 69p.

You could trying posting your problem in the iPad forum but when you get your iTunes receipt I would ask for my money back. The app as not been updated for over 30 months and the reviews are a similar age and a mixture.