I'm a British citizen travelling from LHR to the Hartford, Connecticut airport, and have a two hour stopover in Newark NJ on my itinerary. At what stage would I clear immigration and customs on arriving in the USA?
Generally, you clear US customs and immigration at your first point of entry into the country. In your case, that is Newark.
There are a few airports outside the US that offer "preclearance": you can clear US customs and immigration before departing. Most of them are in Canada, Ireland or the Carribean. London Heathrow is not among them.