Sounds like your issue here Tony:

http://forums.whirlpool.net.au/archive/1223400

That info was current for 3.0 of the OS, but if it was indeed a "feature" maybe it's still the same issue. Sending multi SMS defaulting to MMS so it only costs you the price of one message instead of multiple.

The solution in that thread (at least up to the point I read) was to turn off MMS before sending.