p.4 Barclays conducts on-site audits - but does not say if these are self-assessed or conducted by a third party.

"Barclays generally requires that suppliers who are medium and high risk as they are material suppliers to Barclays must conduct an annual self-certification against our supplier code of conduct. High risk suppliers within this group are also subject to on-site audits every three years"

p.7

"For our material suppliers, we also carry out annual checks to ensure continued adherence to these standards using a combination of detailed attestations and on-site visits. The attestations make specific reference to the Modern Slavery Act and the International Labour Organisation Core Conventions."