The book doesn't give any clues about a specific source or time period. I had never heard of Essex Hotpot before, but looking at the ingredients I would assume it comes from somewhere between the rise of canned produce in the Victorian age to the rationing following WWII.
It is however a very easy dish to prepare and after preparing it for the family I think it will become a regular thing at our house.
Ingredients:1.5 lb of cubed pork
1 can of chicken soup
1 small carton of chopped tomatoes
4 or 5 potatoes, peeled and sliced thinly
Half a cup of grated cheddar.
Method:Chop the onions up and fry them up until they're starting to go translucent. Then chuck the pork in just to brown the meat off. Next add the whole tin of chicken soup and the carton of chopped tomatoes. Place your sliced potatoes on top, and then sprinkle the grated cheese over the whole thing.
Whack it in the oven in a casserole dish for a couple of hours at around 175 degrees Celsius. Easy peasy!