Non-authoritative answer:

[Colin Sabine]

Many thanks for all replies.

I solved the problem by recreating the zone file by cutting and pasting(a
bit at a time) the old zone file contents. After each cut and paste I tested
DNS. I soon found the problem, it was an entry like this:

eagle  IN  A 171.121.167.3
            IN A mailhost

This second line should have been an MX record and so prevented the whole
zone from loading.

Regards
Colin