So for those attributing failure to install as an issue caused by third party anti virus, I uninstalled my Norton 360 restarted and laptop running solely on Microsoft products. Tried installing using windows update and failed for the umpteenth time. I then tried install with wireless switched off as recommended my a post on the tech site and install manual downloaded update file. failed yet again so no further forward other than eliminating the cause to Norton. As I said previously this is a Microsoft issue.
When I updated my desktop which hadn't been done since January the instal was fine even with Norton 360. What I have noticed checking the records is that KB4551762 installed when my desktop version was 1903, then after updating to 1909 the March Cumulative update went in as KB4541335. My laptop history is different as KB4551762 was not installed at 1903 version but trying at 1909 and failing as is KB4541335.
I tried rolling my update back to 1903 to try and start the whole process from my desktop status but would not let me roll back to version 1903.
The only options left is to leave updates switched off and hopefully it will sort itself out or do a fresh install of Windows 10. I will stick with the former as there is no guarantee that Windows 10 might actually reinstall successfully. Next laptop will be an applemac I think.