Hi Barney

Your object is really small just 5mm high. I scaled it up by a factor of ten then it would fillet fine, looked good at 0.5mm fillet. If you need it really small you can always scale it down again.

Actually just tested again if you use a really small fillet it works as well 0.05mm so no scaling required. I would recommend scaling it to real sizes if your object is bigger in the real world these are tiny fillets.

Regards

Jamie