Err, the name of the town seems to have changed from "Somersham" to "Somersham (in progress)". Seems like a weird thing to do to me... :-/

Thank-you, everyone, for your comments. I've now changed to Mercator, and understand projections much better now.

I think I was just confused because the allotments and cemetery - which I now believe to be wonky on the ground- were erroneously appearing as regular rectangles in JOSM.

Aha - if I change the projection in JOSM to "Mercator", then the rectangles are oblique here too.

Ideally, I'd like to use a projection which closely represents what I'm seeing on the ground. So if I walk around a known regular shape, GPS in hand, that shape appears the same in JOSM.

Is there an easy choice?

Thanks for the hint Richard :-)

"EPSG:4326". I assume this is the default, as I don't remember changing it. (But the "might have been drunk at the time" get-out-clause does apply)

I'm pretty sure it was an almost-exact rectangle on the ground, the GPX trace shows in JOSM as a rectangle... so that's how I drew the boundary. But it still looks wonky in the SlippyMap and in Potlatch :-/