Replace wired smoke alarms

[MrMrMr111]

Please as the op, can you get back on topic...

Ok that first picture of the black and grey cable, is actually just black, the grey cable shroud was stuck to it!

So it's
blue & black to (smoke plug side) blue
Yellow to white
(x2) red to red.

I measured and have 240V between blue & black and red, and also 240V between red and yellow.

So do I now ignore the yellow (tape up), and put both reds, and both blue and black into the plug for the Nest Protect?

Thanks