Hi all , and first of all thank you for all the pointers, it has given me the confidence to progress this.
I applied the "24h rule" and came back to this today, and I am pleased to report I have successully disassembled the ratchet wheel.
It turned out to be much easier than I expected.
I started by lining up the square. As Stuart pointed out there are wear marks along the edge of the slot, which gave me the confidence to try a bit harder to slide the washer. I applied pressure at the end of the slot with a piece of pegwood to avoid any damage, and eventually the washer did move and its edge went over the recess.
What became obvious only after disassembly is that the washer was actually dished, and therefore acting as a spring (hence the "no play" I had observed). So it had the ability to flex to go past the edge of the recess of the ratchet wheel. I was initially very reluctant to even try this as I expected a flat washer, which would have simply butted against the edge, no matter how much pressure was applied.
Mystery solved, I have taken some detailed pictures given the interest this thread generated, so I will let them do the talking!