It can work with If fields, yes.
{ = { If { MergeField HasAttorney } = "True" "£82" "0" } + { If { MergeField Day } >= 3 "2" "0" } }
This assigns a given amount to be included in the addition given passing a test. If the condition is not met you could have nothing or have a 0.

Thank you Charles it works

The finished result of what I'm going to use is below, and looking at it now I feel a bit silly its so straightforward.

The result for the most part either being 82 or 164 depending on the database client.

Thank you again!