The first point of contact for most matters to do with the administration of the allotment is the Allotment Administrator who acts on behalf of Merton Council:
Allotment Administrator - Merton & Sutton
020 3876 8812
The service is available Monday to Wednesday, between 9 am and 3 pm.
Merton Council have a web page for Cottenham Park Allotments and the other 17 of Merton's allotments here.
Information about Merton's Councillors is published here. The Site itself is located in Village Ward, but members of CPAGA may be resident elsewhere. One of the Council's Cabinet posts is Cabinet Member for Local Environment, Green Spaces and Climate Change. That Cabinet Member is responsible for allotments. The post holder is identified here.
The Site (and part of the London Borough of Merton) is also located in the Wimbledon constituency of the UK Parliament. The MP for that constituency is identified here, which provides a link to contact details. Members of CPAGA may have another MP. Although an MP may pursue matters on behalf of his or her constituents, be aware that the MP is not responsible for matters that are the responsibility of Merton Council.