Read the decision letter, then read it again
The SIA's letter identifies which criterion failed. Common categories: relevant offences, unresolved criminality, identity inconsistencies, immigration status.
Appeal vs reapply — which route
Appeal to the Magistrates' Court within 21 days if you believe the SIA misapplied its own policy. Costs and legal time make this best for clear procedural errors.
Reapply with new evidence if the underlying facts have changed (e.g. spent conviction, resolved immigration status, corrected identity documents). Faster and cheaper.
Get professional advice for anything criminal
Solicitors specialising in SIA appeals exist. A one-hour consultation is usually enough to know if you have grounds and is money well spent before drafting anything.