The very knowledgeable osiris4isis solved this for me (by PM) so I thought I would post the method in case anybody else has the same problem.
In the primo menus there is apparently normally an option to adjust alert types, this is not available on mine for speed cameras, only for things like railway crossings. It appears that is was disabled for average speed cameras so I added the line

speedcam_soundtype_3=2 ;(1) 1=warn if overspeed 2=approach

to the [warning] section of the sys.txt

Now I get a verbal warning when approaching, the quiet beeps when speed is okay and the loud beeps when I need to slow down. Just what I wanted. Many thanks again to osiris4isis.

Cheers
J D