This part of GOV.UK is being rebuilt – find out what beta means

HMRC internal manual

Claimant Compliance Manual

Cashchceque offences: Investigation by Claimant Compliance Team: What to avoid

You must not, under any circumstances, ask the claimant(s) to provide sample signatures. This would almost certainly infringe the Human Rights Act. If, however, the claimant(s) volunteer a signature to prove they did not sign the receipts for the paid cashcheques(s) you can accept the sample but you must not place them in a situation where they feel under any pressure to do this.