Tools required: Philips screwdriver (PH2)
Cowling = Steering column housing, plastic trim, one large piece below the column, one smaller piece atop.
Four broad, short black screws with black washers (i.e. standard trim screws) affix the trim piece above the footwell (i.e. the piece with the footwell light) - unscrew all four then this trim slides right and towards you and comes away easily
Two short black screws with black washers affix the bottom piece of cowling to the steering column, just underneath the steering wheel, facing you. Unscrew these.
One very long black screw (upper right of the 3 holes under the central cowling) simply fixes the upper+lower cowling pieces together, unscrew this, the top piece should then prise off with minimal force (note the couple of plastic tabs to clip the cowling pieces together again)
Release the steering column lever and pull the wheel down, the bottom piece then pulls away easily to reveal the innards of the column and both indicator stalks
Two small black philips screws hold each stalk into position, unscrew both and pull the stalk away then unclip the cabling plug.
You can then dismantle the stalk itself with another two small black philips screws, and give it a clean inside if it's full'o'muck.
I suspect that one of the two switches (right hand side) is either gunged up or faulty, but I'll clean off the gunge first.
Muck I wiped off (mostly grease to be honest) shown in photo below, but this didn't fix my problem. I suspect one of the two switches is faulty. Need to test the wiring and contacts with a voltmeter next.
Whole process takes about 2-3 minutes.
Putting it all back together again is a bit more of a fiddle, but pay attention to the two cowling pieces which should clip together neatly, and the rubber seals around the stalks sit flush outside the cowling, so getting both in snugly is a bit of a faff, but reassembly took me 5 minutes in total. Screwing the bottom piece back to the column before clipping the two pieces together worked best for me, but ymmv.
NB: I think the shiny screws are holding the ignition barrel in place - it seems I'd have to remove the steering wheel to get access to them anyway.