This is an innocent/ ignorant question Does it apply across makes as well as models?

yes, yes it does.

I've just replaced a Philips twin ( base and second phone) by a Panasonic. Just wondered if I could set up one of the Philips handsets to work with the Panny, but Philips requires a registration code ( shown on bottom of Philips base unit) and AFAICS there is no code anywhere in Panny instructions etc.

Perhaps my thinking is odd.. why buy a twin when you want triple..... but third phone would only get occasional use not worth the extra cost but a bonus.

Yes there is. All DECTS use a code and the handbok normally contains instructions on how to change it (for security).

Defaults are normally 0000 or 1234 or 7890. Try them if you can't find it in the handbook.

you can use them both