The RHS is a great source of advice and information on anything plant or garden related: http://www.rhs.org.uk
The NCCPG - Plant Heritage has a Cambridgeshire group and holds a number of interesting meetings to which both members and guests are invited http://www.nccpg.com
The Hardy Plant Society has a Cambridgeshire and Bedfordshire branch which meets in Biggleswade. There is a good programme of meetings for HPS members - take a look at their website http://www.hpscambsandbeds.co.uk/
Longstowe History Society has an illustrated talk about the History of the Grounds at Wimpole Hall on June 26th - 7.30. All welcome. £3 at Longstowe Village Hall.