Greetings, Mddx,
You will find this a very friendly site, lots of very helpful people.
Whether looking for something specific about your family, or more general such as this topic, someone will answer you if they have the information, or guide you to sources for that information yourself.
Long live Rootschatters (Have you all told someone else about the site today?!)

''I also find that on the census search, clicking on the county at the bottom of the search page after having selected a census year, brings up a list of parishes, and that whilst London appears in censuses from 1871, it is not an option for censuses before that & have found my ancestors in both counties even though they hadn't moved!''
Your point about census entries is good, but since the boundary changes didn't come in until 1889, I don't think that 1871 and 1881 census counties are correct on that site.
The actual images usually make no mention of London or any other county on the page as a whole, or in the description of the enumeration district.
One 'London' borough at random - Chelsea
1871
Individual entries - Chelsea, Middlesex; Lambeth, Surrey; etc
1881
Individual entries - Chelsea, Middlesex; Lambeth, Surrey; also quite a few ''London, Middlesex''!!
1891 census - administrative council now clearly at top of page, London
Individual entries - London, Chelsea; London, Lambeth
However, if you look at entries before 1871, as you say, the demarcation is definitely there. Chelsea is listed under Middlesex and Lambeth listed under Surrey.
Plenty of suggestions from all of you, all I need is a spare hour or two to go through everything!!
Margaret