I just thought I'd follow up since nobody else seems to have had any experience of them.
I decided not to use them in the end. They don't seem to be registered travel agents in Australia, they list two different addresses on their website, and Google turns up nothing about them (negative or positive).
Given that their rates were unbelievably low and the lack of positive feedback on here, I decided it was too much of a risk when booking my holiday.
I might give them a go for a random night some time when it doesn't really matter if it all goes wrong (with my credit card meaning I have nothing to lose).
Al