Can anyone tell me what the plug is that hangs out at the rear of the nearside radiator cowl , not the accessory plug at the seat lock. I've tucked it out of sight but i'm wondering what it's for

If on the left side, then it's the diagnostic plug. Deutsch 4pin in gray with a protective plug in it.

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thanks.
any idea where i'd get a fault code reader and a plug to suit?

IIRC you can get the error codes to show up on the instrument cluster - can't remember how to do it exactly though - think it's something like have the bike switched off with the engine kill switch set to off and then turn the ignition key to on whilst pressing the mode button - but check your owners manual to be sure - or do a search on BadWeb - it's been mentioned a few times on there ...