If anyone is searching I'm pretty sure it was a 'down-and-welded' Supermarine Swift they used for the trials.
The Swift was replaced by the Hunter.
From flight archive:
1971 | 1620 | Flight Archive
However, the department also undertakes research
and development flying under contract to the Ministry of
Aviation Supply, and recent projects have included
aquaplaning trials using a
Hunter and variable stability
research for which a Bassett and, more recently, a Twin
Comanche have been the vehicles.
Also mentioned in this blog:
http://viewfromthecockpit.**********...1_archive.html
The Department of Flight, which operated the Doves, also had a French Morane-Saulnier MS 760 Paris jet for flight testing. The department also operated a Supermarine Swift jet fighter (later replaced by a Hawker Hunter) for aquaplaning trials carried out on the main runway. To facilitate these trials the runway was checker-boarded with rubber dams which formed artificial lakes. Cranfield also had a gliding club and in addition the Department of Flight operated a Training Flight using Auster Aiglets. This was my idea of heaven.
In place of the *** use
b l o g s p o t/2009_07_01_archive.html (without the spaces)