While I'm at it, where's best to source them? Google is not my friend.
Not anymore. Barton's unfortunately is no more. RIP. First bike shop I went into at age of about 13. Used them on and off for the last 30 years. Tried to book an MOT mid-April and found out they'd shut at the beginning. Rather pissed off because I always had it in the back of my head that I'd buy into there once I stop doing the corporate thing. Probably a lucky escape. So for MOTs I'd now recommend D&H Motorcycles.
Quite. The proprieter is now running a mail order business (I think) but it's a sad day (well it was). Barton Motors was set up as the retail arm for Douglas Motorcycles at some point between WW1 and WW2.