I have bought two (used) Hispec calipers (ultralight 4) that needs this size of disc.
I think the standard cortina disc is 245(ish) mm.

Does anyone know where I can find the above discs?

I use Peugeot 405 solid discs which, from memory, are 265mm diameter, but can be machined down to suit.

Call roadster

Aren’t Sierra XR4 disks 260mm

Good idea!

I shall have a look.