Keyboard printing Alternate characters intermittently

I had this problem before and after every conceivable trouble-shooting step, the Tech at London Drugs ( Apple Service Centre, here) determined the Accessibility/ Virtual keyboard was turned on! At the time I was sceptical that that was the problem, but turning it off seemed to resolve the issue.

However, several times since the Keyboard starts printing Alternate characters and I have to re-boot the system to clear it.

I am using my previous iMac Apple, keyboard and Trackpad which run on Replaceable, rechargeable AA batteries.