In the late '80s there was a trial of hydrostatic drive (EXPT No. QAI 11701) fitted to some units, possibly just to some units at Reading depot. They were supplied by CovRad Heat Transfer Ltd, type CRD 49404.
These views are of a drive that turned up as a spare some years later in preservation at the Chinnor & Princes Risborough Railway. Phil Capron.
They had a more modern cross flow type core rather than the tube type, but the biggest difference was the use of a hydraulic pump which would have a thermostatic control so the speed changes with radiator temperature rather than the right angle drive which ran at a fixed ratio of engine speed.
Class 117 DMS 51397 was one car that received the experiment, which still had them fitted when it entered preservation (below).
(Information on this page gathered from postings on 1st-gen Yahoo group March '09)