I updated the OS on 1st October and had problems for the next two weeks. As it happens it wasn't the OS causing the problem but the graphics card. Apparently this version of the graphics card has had issues and Apple had extended the warranty which had expired on 30th September 2016! After advice from AA | Mac (the company looking into the problem on my Mac) I 'phoned Apple and they agreed to cover the cost of the replacement despite being out of warranty. What a result - thanks Apple
So I am now up and running again!