Thank you for your advice. I've had a look to see if any events are present and, although I have done some automation in the clip I'm adding the pasted one to, there are nodes at the end, ending any automation. When I check the midi track, before pasting, there doesn't seem to be any CC event activity. When I drag the pasted clip into its new position, a new sub-track appears under the midi track titled "A1". As soon as this appears, the volume for the whole midi track goes down. I've tried removing it and it doesn't change anything.
I've found that when I copy a clip from another project, and paste it into a new one, the automation is in tact and the the problem with the volume doesn't happen. In this instance, the A1 sub-track doesn't appear when I re-position the clip. Instead a new full midi track, with the automation lines, appears at the bottom of my clips, titled "Track 5" (or whatever number is next). That's only difference I can see between it working fine and not. Sorry for the long-winded explanation - I'm confusing myself now!