Hi Bruce,
Thanks for that - I'm wanting to insert silence between sentences - using your suggestions I've found out that by having another empty track open - making a selection then shift clicking in the Marker Track - Copy and Paste in Shuffle mode at the appropriate place in the Audio track - that does the trick.
Using Insert Silence from the Edit Menu doesn't work.
Anyway I'm now able to do what I need to do and that's the main thing
Thanks again, Colin