Went through it all pretty much folder by folder and noticed lots of randomly named folders here there and everywhere. Ran a check in Defraggler and it came up fragged to hell with about a dozen unused sectors. Ran TRIM a few times through ssd toolbox and got back to 12GB free.
With seeing all those junk folders in there I thought why not give ccleaner a go, it's been running almost 5 years without running anything like that. Ran a check with that and cleared out 42GB, lol!
Ran it another couple of times to be sure, last one with "wipe free space" enabled, and voila, windows finally showed what it should, 55.9GB free of 111GB.
So I think it just needed a good clean out in the end, should probably do it more often in future, lol!