One light has just dawned.

Mozilla::LDAP::API give some (self confessed poorly documented) access to the 'C' libraries underneath.

Not a lot of good with the documentation as shipped, but I happen to have a copy of the Tim Howes/Mark Smith LDAP book.
I had discounted this as it was firmly aimed at 'C' programmers, but coupled with the API it may give me a way forward.

I am still no closer, however, to working out why the session kills itself after delivering the data.

Cheers

Dave R.