Please can anyone help me.
Total novice here who bought his first bonsai tree and have only had it a week.
I have watered it once using the plunge technique for an hour.
Not sure if I have the location correct as it’s not in direct sunlight.

If you use the search function and search for "Elm yellow leaves", then you will get allmost 200 hits.

Elms usually get yellow leaves when the tree isn't watered enough. Try keep the soil moist all times. The soil may not dry completely and not be wet all times. I looks dry.

You should read about caring for an Elm. Otherwise we will see you soon again. Good luck.