Roll up folks, we have a brand new and completely unused Shimano Hydraulic rear caliper from the GRX 810 series.
£50 plus postage or pickup in London seems fair.
A great caliper, perfect for your gravel build. I think it was Reginald Gravel himself who pronounced this 'a ruddy good caliper'
The details, from Shimano themselves:
SHIMANO GRX BR-RX810 DISC FLAT MOUNT CALIPER - REAR
Engineered for performance over mixed terrain - be that tarmac, gravel or hard-packed mud, Shimano's GRX BR-RX810 Disc Flat Mount Rear Brake Caliper quickly dissipates heat and clears dirt so you get consistent braking power. Setup is simple and ongoing maintenance a breeze thanks to a clean bleeding system so you'll never hesitate to tackle the roughest surfaces.
Great control over mixed-terrain riding
Excellent heat dissipation
Flat mount and anti-mud specification
Convenient tool access
Simple wheel replacement
Easy and clean bleeding system