What is this over-inflated rugby ball?

Guinea fowl

Ah, thank you! I hope it comes back.

An escapee.

It's back! Do I just leave it be? It doesn't appear to be tagged and it looks quite content, squeaking to itself like a wobbly trolley wheel. I had a quick search for lost birds locally but couldn't find anything.

It's had a dust bath, a bit of sun and is looking for scraps under the bird feeder. For a bird that is apparently so unintelligent and timid, it holds its ground against the squirrels better than the many regular pheasant visitors.

Yeah just leave it...could be a bird reared for shooting if there is such an area closeby to you or as tuwit said an escapee from a collection or third option deliberately let go cos its no longer wanted

They are brilliant for Tick control :) and also make good guard birds as they raise the alarm over predators