Quantities will need to be adjusted if you use U.S. gallons. The recipe comes from Dave Line's Brewing Beers Like Those You Buy. Water for bitter brewing means hard water.
Invert sugar is sugar that has been cooked for a couple of minutes over a low flame.
Isinglass apparently comes from the sexual organs of certain fish. Contain the mashed wort in a large grain bag to retrieve the sweet wort. Dissolve the main batch of sugar in a little hot water and add this during the boil. Also pitch in the Irish moss as directed on the instructions. Switch off the heat, stir in the second batch of goldings and allow them to soak for 20 mins.