I had an accident 2 years ago - other driver drove into the back of my car when I was totally stationary at a giveway, so 100% their fault but of course no witnesses and they made up a *** and bull story which eventually led to me having to settle 50-50

That was bad enough but I now learn they are also making a big claim for uninsured losses as well so looks like I am going to be seriously out of pocket too

What should I do, and how is this 'uninsured' part allowed to happen??

The most common 'uninsured loss' is the excess the individual may have had to pay on his insurance policy. Others include car hire, time off work etc. It basically means costs he incurred as a result of the accident which his insurance company did not cover. You need to get a fully itemised breakdown of all these alleged losses and make sure he has supported documents, receipts etc. Check with your insurer and see what they say, also check whether you had legal expenses cover.