Hi Angus,
it might sound strange but you can use almost any cheap el-crappo telescope as a guide scope.
Many people even use a finderscope.
A Guidescope does not need to be optically excellent. Nor does focus need to be sharp, which is actually a benefit as guide stars work better if they are slightly out of focus.
The only thing I would suggest is don't go any slower than about f7
The faster the better. Some cheap achro's are available around f5
I've even seen people guiding with $50 scopes picked up at Garage sales for $10