Thanks guys,

Worked it out, what had happened is 2 of the 6? Telstra nameservers had
picked up the ns2. record without the ip number (why it was there I have no
idea) whereas every other nameserver seems to have ignored it and picked up
the previous line ns2.seol.net.au 203.220.28.7.

I must admit I am now confused by "glue" records, I always have believed
they were entered at the Registrars end ... and I had fallen for the three
card trick and not looked close enough at our DNS to resolve the problem.

Can anyone shed some light on this?

Cheers Tony