Key changes on keyboard

[Rangefinder10]

I am using an Apple wireless "British" keyboard set-up on an Imac running on OSX 10.9.5.

Recently my @ and " keys have changed places. It took me a while to identify where these errant keys were.
Presumably I must have done something to cause this, my problem is that I have no idea what this might be.

Any ideas how to correct this??

Suggestions gratefully received.

Regards