OK, I've built in the various comboboxes and now have this:
SELECT Cases.Case_Type, Cases.Funding, Cases.Litigation_RAG_risk_rating, Cases.CMP, Cases.FCO_lawyer
FROM Cases
WHERE (((Forms.frm_casefiltersedit.cbo_casetype Is Null)
OR ([Case_Type]=Forms.frm_casefiltersedit.cbo_casetype))
AND...