Hi,
I believe the assesments take out around 70% of people so the number for interview will be quite low.
In terms of preparing for interview you need to take safety, all the time. You MUST come across as someone who thinks constantly about safety.
Secondly, a good knowledge of the company and job is essential. DO NOT however simply look up Wikipedia or the like and rhyme off whats there. Do you know the MD's name? How long the franchise has to run?
However, the very best thing you can do is visit the depot at Dumfries and ask for a look around. It isn't normally a problem and then you can ask about depot related things, learn about SPAD's etc and generally have a knowledge of the depot. It may even be that you get to speak to the manager doing your interview which would really help you. Lots of people do this so don't be shy, just go along and ask. A depot visit is an absolute requrement for internal candidates and if externals can do it is always goes down well.