Chris wrote "...........I suppose they are normal ones,they were
given to me a few years ago as pot plants which I put in the garden when
they had finished blooming......."

That explains a lot Chris
The flowering time of chrysanths is governed by daylight length. Most of the
varieties used for pot production are different varieties of the "Princess
Ann" group, which are naturally November flowering.
Pot Mums are treated with a growth retardant, which keeps them compact, but
once planted out into the garden the roots get into fresh soil and the new
growth of the plant will grow naturally and will mostly get to around 4ft.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk