In message
Rod wrote:

... Mention of HVDC reminds me that the Channel link is one such (at
least, I think that's what I remember). So that prompts me to ask, how
closely synchronised are the UK and France, and indeed the other
European countries with each other?

UK-France, like all lengthy underwater/undergound (i.e. high
capacitance) links, is DC, so obviating any need for synchronism.

Don't know about the rest of Europe, but inter-system links elsewhere,
even when 'overhead', are often DC primarily to avoid the need for sync.

--
Peter Duck