Thanks @nordiclondonmum. I couldn’t get anything specific out of DC but the nursery will give you an idea of what to expect eg recognise their name, write name, count and recognise numbers, use scissors, jigsaw puzzles (up to 16 pieces or thereabouts), listen to a story and answer questions, draw a picture of mother or something else, talk about their family (family members, what parents do for a living, what activities does dad do with DC etc), where they live, build something with LEGO or blocks.
The most selective north London schools are:
Girls - SHHS, NLCS (though not in London, they have an extensive coach network), Habs girls (again not in London, I believe they are trialling a limited coach service with Habs boys but not entirely sure).
Boys - UCS, The Hall (must register before your DS turns 2), Habs boys (see above re coach service).
Co-ed: Highgate (you enter a ballot to sit the assessment), Trevor Roberts (they have a 1-2-1 assessment but not sure how selective they are- you need to register very early though and you have to do a tour first).
There are schools that are very good but not as hyper selective eg St Anthony’s, Channing etc.
Hereward House is also very good and don’t assess the boy; it’s only an interview with the parents and they ask for a nursery report.