Dropdown conditional upon two fields

I have a requirement to provide a dropdown list on a form which comprises a filtered list of entries in a second related table. The conditions for appearing in the dropdown are;

a) That a field 'Active' is checked on the record in the secondary table
b) That a field in the secondary table matches the entry in a field on the primary table

I have a many to one relationship from the first table to the second. I am displaying the related field on the form and setting the conditional values to the matching field b) which works fine. However i cannot seem to find a way to also filter out records which are not 'Active'.

Thanks for any help with this.

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Kevin Plant
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