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