A large, well-formed rosette

To start this blog I have posted a picture of a large (13cm/5in across), well-formed rosette.

I have searched far and wide but cannot decide what it is. In some ways it resembles a cultivar called 'Sharon's Pencil', but I am far from convinced.

Since I bought it in Hastings last autumn, it has grown slowly, but still shows no sign either of producing offsets or flowering. If anyone has any suggestions as to what it is, I would be grateful to hear from them.