but what about the rcd's I got from Screwfix. They are plug in
types to replace re-wirable fuses on old ccu, only plug into
phase conductor

I assume you mean RCBOs, which are RCD and MCBs combined which can replace a
fuse.

They work because they also have neutral connections. A flying lead
(sometimes supplied hard wired, sometimes with a Supply Neutral screw
terminal) goes to the neutral bar. Both the Live and Neutral from the load
connects to the load terminals on the RCBO.

If you don't mean an RCBO, please post the screwfix item number so we know
what you are talking about.

Christian.