Thanks pennbank but I did do this and that was what, I think, caused some of the problems.
My Apple person said that although the files were in trash they might still be affecting the re-install and so advised emptying the trash in order to completely clear them all. And of course that's when we discovered the problem with the non-emptying trash can! Once that was solved the re-install worked a treat.
So I for one will be emptying my trash very often!