Presented in the now-traditional illustrated form:

; sdig ns gcluk.net
ns1.gcluk.net.
ns2.gcluk.net.
; dig mx web1.gcluk.net @ns1.gcluk.net
[...]
;; ANSWER SECTION:
web1.gcluk.net. IN A 65.98.36.90
;; AUTHORITY SECTION:
web1.gcluk.net. IN NS .

(TTLs have been omitted for clarity.)

That's an interesting authority record they're returning; somewhat more like a disclaimer of authority record, in fact.

The effects are closely related to what I saw in HowNotToDoDNSVII . In this case, we first looked up the A record and found one, but when we tried to look for an MX record the completely bogus NS record stymied our attempts. (Among other problems, there is no A record for the ' . ', the DNS root.)