Just an update on this. I have managed to resolve my issue. Thought i would post some info here to help anyone with similar questions.
My issue turned out to be related to the fact that my form is multi page and the variable I was using to hold the data received from the SQL table ($form->data['dbdata']) gets killed when the form goes to the next page. I am not sure how to prevent this but managed to work around it another way allowing the sql query code to be run on the same page as the combobox group. I’m sure someone else will know how to get around the issue properly.
Anyway for info for anyone else doing the same thing with the example above you need to put the following into the dynamic data fields of the checkbox:
value key= es_equitmentid
Text key= es_equitmentname
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