'http://datapoint.metoffice.gov.uk/public/data/val/wxobs/all/json/sitelist?key=[my key]'
But when I attempt to obtain observations for a specific site (from the site list)
'http://datapoint.metoffice.gov.uk/public/data/val/wxobs/all/json/3535?key=[my key]'
I receive a 404 error on the IDs I have tried.