Thanks to osiris4isis & LouBouha for the replies.
I don't think the problem is with the TTS as I don't even get the "beeps". Just to confirm that the TTS works okay I temporarily change the fixed speed cam entry
[speedcam_category:0]
activated_spoken_type=speech
activated_speech="Caution speed camera ahead"
changed to
activated_speech="Caution average speed camera ahead"
Now when I approach a fixed camera I get the beeps and a spoken message "Caution average speed camera ahead" so I now know it can say the message fine. The problem seems to be that is is not being triggered.
With a non-TTS voice selected I get the warning beeps for fixed cams but nothing for average speed cams
I am now wondering if the [speedcam_category:3] is the correct category, I have noticed that category 5 & 6 are not in the sys.txt. The speedcam database must have the correct categories as it triggers the on screen icon with the current average speed below.

Thanks
J D