Had the same issue on my old

1.9 petrol.
One of the previous owners had changed the solenoid but the problem persisted when I took ownership. Got some good info from BY about cleaning all the earthing points starting with the starter motor- thoroughly clean the starter where it mounts to g/box, (the ring and bolt mounts) get all the crud and paint off.
Next clean the g/box where the starter inserts. I used emery paper and a sharp scraper for both.
Next do the g/box body earth strap. Not sure where this mounts on the 2.1 but the td goes from g/box to the mounting hanger and I recently sheared the bolt here (oh joy !

) so you may want to go careful with it.
Next one is the battery body earth strap. Also do the obvious and make sure the battery is kicking out enough
Did all the above separately and the "click" lessened each time. Although Im not 100% sure if it was totally cured, you'll have to ask Twiggy. Shortly after doing the last bit he wrestled me to the floor and beat me with sticks until I sold it him
Good luck with it