sgrobinson wrote

Are they compatiable?

Yes, they are compatible to each other. They even use the very same firmware (despite version numbering), meaning you can install the v2 firmware on a CPE510 v1 (valid for CPE510 v2 firmware only).

I'm in the UK and a CPE510 V1 with V1 works, but a V2 with V1 doesn't appear to work. Possibly as it's missing the 'Region' setting?

If your CPE510v2 has been bought in the EU, it uses the EU region automatically - no need to set region anymore. You just need to set the CPE510v1 to region "UK". Note, that albeit you can install the v2 firmware on a v1 device, the CPEv1 still will offer region settings even with v2 firmware while the CPEv2 does not.

Maybe it makes sense to install latest firmware 2.1.11 from January 26, 2018 on both devices.