tax on import of services

dear members, need your help to understand if we are liable to pay GST and if yes, what is the taxable amount. we collect
money on behalf of foreign institute, and to that money we add a small amount as our admin charges of 15%. so hypothetically we collect 5000 from which 4200 needs to be transferred to the UK bank account. so need your help if we need to pay GST and on what amount..thanks.