Strange how the chainwheel is more worn than the sprocket - but maybe it is alloy while the sprocket is steel?

If it's the original chainring supplied by Thorn it is almost certainly aluminium alloy.

With teeth starting to hook is there a risk of the chain being sucked up the back of the chainwheel and locking up?

Very unlikely on hub gear without a derailleur or derailleur type tensioner. Even with a slack chain, though with the latter it can drop off the chainring.

You might eek out another few thousand km's by taking a link out, but the failure method becomes more and more likely to be a snapped chain.  I'd consider I'd had good value from it. flip both chainring and sprocket and fit the new chain.

Agree. After 22000 km and given the wear on the chainring, time for a new chain and flip the sprocket and chainring.

Snapping a chain could be very inconvenient.

On a hub gear bike I have had this happen once, the outer plates on one link failed on a very worn chain, but nothing like 22000 kms use. This was before I started using Chaingliders, which make a significant difference to transmission maintenance and longevity, at least for all-weather use in the coastal area where I live.

Fortunately for me I was only a few kms from home and the bike trailer I was towing had been unloaded before the chain broke. So not too hard to push the bike/trailer up the hills and coast on the downhills. My bike toolkit now includes spare 1/8" chain links in addition to the 3/32" quick links and spare links I already had.

Hey All!

I came across this service today...

https://eclean.green/collections/top-products/products/down-sleeping-bag-cleaning?variant=46105236242629

That sounds interesting, I've heard of dry cleaners offering chemical free cleaning suitable for down, I suspect it's the same thing, though have no experience.  Thankfully my experience with traditional down cleaners has been the opposite to yours, my down quilt comes back looking and lofting like new.  I note the link above says it saves 80 gallons of water, I doubt the home washers use half of that.  In the end it's what someone is happy with, in terms of both results and risks.

You might eek out another few thousand km's by taking a link out, but the failure method becomes more and more likely to be a snapped chain.  I'd consider I'd had good value from it. flip both chainring and sprocket and fit the new chain.    I don't always get the same distance out of the second side of a chainring and sprockets, maybe 70%, but yours don't look as worn mine usually do at that mileage.  Even at 70% that's nearly 40k km from a chainring, sprocket and two chains, that'll turn most people running a derailleur green with envy.

I take it from what you say that the slack in the chain has already reached the point where adjustment is necessary but there’s no adjustment remaining on the eccentric.

Yes, and it's come off a couple of times now. The chain has never been replaced. I did buy a replacement at some point but haven't swapped it yet, but from what I understand now would be the time to do so.