Live HTTP Headers gave me the clue; using the std nagios check_http http://nagiosplugins.org/man/check_http I just amended the -u option to specify the fuller path to the frame instead of just the front page & it worked like a charm.
:)
I owe you a beer ...

Cheers
Chris

PS If there is a way to mark this as solved, please do. In the meantime I'll edit the title of this reply