Multiple letters are typed when one key is pressed

On my Apple Wired Keyboard (the latest one - came with a 2012 Mac Mini), if I press one of several keys, it acts as though multiple ones are pressed.

As an example:

If I just press letter b:

;§ appears. I don't even know how to get that symbol, other than by searching through special characters.

and held down:

mbwvzjmbv§wmwvzxmjv§

With letter f pressed once:

RLF?

When letter f is held down:

'yf L?g.crl . .cr.PY?cg.g .lg

When letter d is pressed:

a or s-

When letter d is held down:

dheeens-haeoaaaehtns-htns-

The down arrow seems to want to toggle Mission Control and Volume key - I don't want to do it now for fear of messing my system up.

It happened once in Yosemite Public Beta and I had to re-install OS X. It's happening again, and because it is happening for a second time, I wonder, does anyone else have this problem?

If so, is there a fix for it? - I sent a bug report in the feedback tool but it doesn't seem to have been fixed. As far as I know I haven't installed any dodgy software.

Currently I'm typing on my ancient black translucent G3 keyboard - far from ideal!

Problem occurs on my iMac; mid 2007, 20 inch.

Cheers,

Lyndon