I have a Apple TV 3rd generation whilst testing the sound through my arcam amp via the optical lead I noticed that the Dolby symbol didn't show on the amp whilst watching a few trailers for films, is this simply because I'm wAtching a trailer and only available whilst actually purchasing the film?

If the amp was receiving a Dolby Digital signal but thought it was plain vanilla stereo or the like then what you would have heard would be an unholy mess. (assuming the amp didn't simply mute in protest) So if it sounded OK then either the Dolby light on the amp isn't working or it wasn't a Dolby soundtrack. My vote is on the latter.

The Dolby is clearly working ad when I have the sky or blueray plugged in it displays Dolby or DTS

I've just done a test on mine and the trailers do not appear to be in Dolby Digital although films that I have bought/rented are.

The info page of movies will show the Dolby logo if it is Dolby Digital.