Bertie,
We always rent an apartment privately from a Portuguese guy.
In Cabanas itself you have 3 places that you can find on the web.
Cabanas Park complex which is just newly opened, Golden Club or Pedras da Rainha.
You should be able to see them and get prices for when you want to go.
If you want to go privately, then I suggest you just do a general search on apartments to rent in Cabanas. I often do this and have seen lots of different ones and varied prices.
As you have lots of time before you go, you should be able to find something no problem.
Very often in the supermarket in Cabby there are adverts for apartments and if you like, I can get a couple of phone numbers for you when I'm there in 18 days!!!!!!!!!!
If I can be of any more help, just ask!!
Pat