Went to Covent Garden to See and Oritorio of Alexanders Feast With the Coronation Anthem Violin Mr Hays organ Mr Stanley the King and Queen & all the Royal Family was their and Cost me 5/-
Alexander's Feast is by Handel, of course. This data base shows that there was another performance on the 25th with the same performers. It also tell us that Thomas sat in the first gallery, presumably what would be called the Dress Circle today.
And here is a link to tell you about the Coronation Anthem including a chance to hear the music!
There is a picture of Covent Garden Theatre at the right sort of date here.
The Monarch remains George III, pictures of him & Queen Charlotte here.