OK, I've found https://help.openstreetmap.org/questions/731/how-can-i-revert-a-changeset
I'll give it a go.

Phew, no, this is the only Elgin area change set affected. Looks like I did them all in one hit.
(Still needs tidying up mind you - e.g. at least one of the "car parks" is someone's front garden, but someone will need to go look to confirm - I'll add a note once we've reversed the changes out).

Bugger, yes, I did. Apologies.
OK, how can I reverse this lot out?
(I think there's also another smaller changeset in the same area which will need reversing out too - I'll have to check back.)

Apologies, wrong source. Actual source was county council online definitive map at http://my.cambridgeshire.gov.uk/myCambridgeshire.aspx?MapSource=CCC/AllMaps&Layers=row,row-tab=maps