It was suggested that due to the length of coax I was using (4m)… there wasn’t a need for a counterpoise with the particular antenna I am using. 
I’m very keen to test different things (counterpoise, ground, choke, battery)… one at a time.
A station (or two) on 10m willing to test/figure things out with me would be fantastic… but I I think I need to be on a summit (home QTH isn’t great).
Feedback from @W4GO :
Yesterday, @SQ9IDE reported an audio/modulation issue on 10m.
I’ve had an EA station suggest I check my battery (modulation issue) but I can’t remember what band.
On the Trans-Atlantic day (on 10m), a UK station whom I know worked me whilst on the summit , and reported an issue. I switched to medium power - the issue was still there, but to a lesser extent. I also changed the radio’s position relative to the antenna.
This station suggested that RF might be getting in from the mic, hence I changed the cable. At least now I know my issue is only on 10m (nobody complained on 40 or 20m on the last activation.
I’m happy to head to my local summit (Staple Hill) and take a load of stuff with me (including a lead acid battery) to work one or two willing stations who might help identify what it might be. 