Gnasher42

I thought I would follow up with where I have got to with this issue, perhaps to help anyone who has similar issues. Following up from MEB's suggestion that there is a double click happening hone there should only be a single click, I further investigated the hardware. Swapping back to an Apple Magic Mouse and the Wacom tablet and disabling the 3Dconnexion mouse at least partially resolved the issue. Affinity Designer was working fine but I was still having problems with other applications with missed mouse clicks. Web searches on the 3DConnexion mouse suggests that there have been problems with this issue in the past. I contacted 3Dconnexion and they directed me to upgrade the firmware in the mouse - which can ONLY be done using a Windows computer. I am a Mac user but, fortunately (or unfortunately) I also use a windows installation on the Mac and completed the upgrade that way. This has resolved many of the issues with AD and with other software but I was still getting weird random missed clicks and occasional double clicks when there should not be. 
Final result: disconnect 3Dconnexion mouse and uninstall driver software. Result: Working Affinity and 3D software (Fusion 360) and no more problems. I have found that a combination of Apple Trackpad and Magic Mouse works fine with my 3D software and my very old Wacom Intuos 4 tablet plays very nicely indeed with Affinity software!