"Mr Dell" finally arrived this morning complete with replacement mother board, pleasent chap, didn't mind silly questions or someone looking over his shoulder, perhaps the coffee & biscuits helped !
Made sure he observed the issue before launching into MB replacement, he certainly had not seen this fault before and neither of us believed that it was a hardware issue, however, for want of any other suggestions, went ahead and replaced the mother board.
Thankfully, we were both wrong, the fault was indeed completely cured.
Only action required was to re-load the BIOS, was already up to date but is a Dell requirement to do with any future recovery requirements after hardware changes have taken place.
Windows was still intact as the SSD was transferred to the new MB, however the new key provided was registered on line as there can be an issue with Microsoft authentication when major hardware changes are made to the PC..
Again, many thanks to all forum members who showed interest and offered suggestions, much appreciated, a good result in the end, certainly worth note for future reference.