[Hilary]

My friend was given a stalk of this plant  last year, stuck it in some earth and has this large flowering plant this year.
Any ideas of its name?

[Miriam]

Ruellia brittoniana.

[Alisdair]

Thanks for that, Miriam!
Usually we stick to Kew's The Plant List for nomenclature. The Plant List holds that Ruellia brittoniana is a synonym for Ruellia simplex - but (as you can see by clicking here), Kew's partner in compiling The Plant List, Missouri Botanical Garden, are sticking to Ruellia brittoniana. So I think we should too!
And indeed, Hilary, your plant looks like the cultivar 'Purple Showers'.

[Hilary]

Well thank you both for your quick information.
I sometimes feel cheeky asking for names of flowers when I could spend untold hours searching books and Internet.
Many thanks