Have investigated the indicator canceling mechanism tonight. There are two separate bits to this, which I'll describe briefly in case anyone else comes across this later.
There is a plastic arm attached to the indicator stalk with two small springs on either end. Normally this sits inside the back of the steering wheel hub but when the indicators are on, it protrudes outside the hub and is caught by one of two lugs on the hub to spring it back into position and thus cancel the indicators when knocked from the right direction. Mine is missing one of these springs, and thus has no cancel facility at all after turning left
The second part which causes my stalk to cancel after a right turn, but then move all the way down to indicate left, is the curved notch on the stalk which locates against a sprung ball bearing and gives the stalk its up, centre and down positions. The curve is quite worn on its top side, hence when the lug on the steering wheel hub cancels a right turn, there isn't enough resistance to catch the stalk is the neutral position, and it moves right down to the left turn position.
Need a replacement!
.....which will presumably include the cancelling mechanism complete with springs, will it???
The horn does have two spade connectors on it. At least one goes into the wiring cluster, haven't traced the other yet as its an arse to get to behind the bumper.