Green’s Dictionary of Slang

wanstead (flats) n.

[rhy. sl.; ult. East London area Wanstead Flats]

spats.

H. Champion ‘Haning ’Em Out to Dry’ [monologue] Once I went to the Serpentine [...] I took off my boots and started soaking my Wanstead Flats.
[UK]R. Puxley Cockney Rabbit.