In answer to the question "Do you know the location of the Sequoia (Redwood) tree?" there are two - see details and photos at
http://www.redwoodworld.co.uk/picturepages/hampstead.htm .
Worth exploring hampsteadheath.net for other places of interest to try to find on Hampstead Heath, e.g. https://www.hampsteadheath.net/landmarks and https://www.hampsteadheath.net/ghost .
And plenty of other places not listed there, e.g. the old orchard, the hollow tree, 4 of London's 13 protected views[0], 2 old ice houses, the old duelling ground, etc.