I am afraid that both my claims were wrong:

that the NS records in the zone were different from the ones
at the delegation point (I had picked the wrong set of NS
records from the authority section of a reply)

that mail1.ccs.bbk.ac.uk was not authoritative for lkl.ac.uk
(its not authoritative for bbk.ac.uk which the CNAME points
into, but so what)

That will teach me to post too fast (maybe). Apologies.

--
Chris Thompson
Email: cet1@cam.ac.uk