Sorry to bump this old thread, but I guess this bug is the cause for the problems.
Compare the output of dig for TXT and SPF:
~# dig TXT wilson-house.co.uk
(snip)
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 44916
~# dig SPF wilson-house.co.uk
(snip)
;; ->>HEADER<<- opcode...