Quadrant looks similar to ours (although mounted upside down, which is interesting but probably doesn't really matter). To remove, first remove the steering wires, it'll be much easier while the quadrant is still held in place. Undo the double nuts on the end of each swaged stud while holding counter so the stud doesn't turn. You may also have to drift out the small pins retaining the wire on the far corners of the quadrant so the wire can be freed, as it doesn't look like the studs will pass through there otherwise.
One difference from ours is the single bolt protruding at the front. It looks like this may have be used instead of a key in the shaft. If so, or in any case, undo the bolt about 1 cm or just take it out entirely. Mark the vertical position on the shaft with a black marker (although probably it's left enough marks from clamping anyways). Then undo the clamping bolts and the quadrant should come off (tap with rubber mallet if not).
When refitting, make sure you put some dielectric grease between the shaft and quadrant. Then put the steering wires back in place, center both the rudder (emergency tiller is useful for this) and the wheel and then adjust the nuts on the studs until both wires are slightly tensioned. Then lock off with the double nuts. Check the steering goes easily and has no play.
While you have the quadrant off, check the large lip seal at the top of the (black rubber) rudder tube for wear. Mine was in a dire shape and needed replacing when I did this job. Standard part from an industrial supplier, make sure it's got a stainless spring though. I also used the opportunity to repaint the quadrant (aluminium etch primer + black Hammerite).
Picture of ours (I expect yours looks like this on the underside), with the large key still in place. You can also see the very worn old lip seal:
