GR4 Techie,
The Chipmunk with the unusual engine cowling, used for aerotow at Bicester was probably the "Supermunk" that had been modified by replaced the gypsy major engine with a rather big Lycoming ISTR. I got an aerotow from it one breezy day, whilst Flying Bicesters ASK18 (light glider), after the usual "up slack". All out was signalled, the throttle was opened and I took off in his propwash. Grit my teeth and hang on was all I could do. Hope that helps. It wouldn't surprise me if Coff has a photograph for you!
Smudge
PS GR4, this could be the super Chippie you saw. I believe it saw service with the RAFGSA at Bicester;
Credits as on image
Smudge