I thought these might be a useful guide - I don't know how accurate they are but have checked them on a couple of sites and they all seem to be quoting the same dates.
WINTER SPRING SUMMER AUTUMN
ZONE FROM TO FROM TO FROM TO FROM T0
A 13-Feb 27-Feb 16-Apr 02-May 03-Jul 01-Sep 22-Oct 03-Nov
B 06-Feb 20-Feb 06-Apr 25-May 03-Jul 01-Sep 22-Oct 03-Nov
C 20-Feb 06-Mar 19-Feb 07-Mar 03-Jul 01-Sep 22-Oct 03-Nov
WINTER SPRING SUMMER AUTUMN
ZONE FROM TO FROM TO FROM TO FROM T0
A 13-Feb 27-Feb 16-Apr 02-May 03-Jul 01-Sep 22-Oct 03-Nov
B 06-Feb 20-Feb 06-Apr 25-May 03-Jul 01-Sep 22-Oct 03-Nov
C 20-Feb 06-Mar 19-Feb 07-Mar 03-Jul 01-Sep 22-Oct 03-Nov