Within the next few weeks I hopefully will have a new Cairn Terrier puppy and it has been suggested that I use a pet cage especially in the back of my 4 x 4 when I am travelling.  Having never used one please can someone give me some advice as to the size and type I would need.  Also when I should I use it apart from in the car.

A car cage is a good idea. My friend had a car accident and having her puppy in a cage saved it's life. You can get cages from almost anywhere these days. You might like to have a "made to measure" cage for your vehicle.
A cage for the house and suitable for the car must be big enough for him when he is full grown. In the house you can make the cage like a little den for him and he will like to escape into it. My son-in-law didn't believe me when I suggested this to him but having had a destructive couple of dogs, he soon learnt it was a good idea. What amazed him was his dogs liked their cage so much, that if the door was left open they would spend all their time curled up together in it!
I am sure somebody here will be able to tell you where to get these cages.