We are planning on growing some dwarf Cannas in containers this year and
would be grateful for advice.

Having done some research on the web there appears to be some
conflicting advice around (for a change). Hart Canna suggest planting
individual rhizomes of the same variety 6" apart in groups of 3 or more
whereas http://faq.gardenweb.com/faq/canna/ suggests 12" - 18" apart.
Perhaps the latter is for the large specimens?
The varieties under consideration are Louis Cottin, Aranyalon, Golden
Girl and Sudfunk.

As far as we can determine we should buy the rhizomes now and place them
in compost (Hart indicate peat/coir compost) and keep them just moist in
a frost free environment before transplanting them into fairly deep
exterior planters containing a rich compost (JI No.3?) when frost is
unlikely.
--
Robert