If you're just starting out in the world of PAYE software you may find it useful to check out these documents,
- Notes for payroll Software developers - http://www.hmrc.gov.uk/comp/index.htm
- Quality Standard, detailing how each field within PAYE End of Year Returns should be completed - http://www.hmrc.gov.uk/ebu/qual_stand_10.pdf
- Calculating Tax - http://www.hmrc.gov.uk/ebu/payerout12.pdf
- Calculating National Insurance - http://www.hmrc.gov.uk/ebu/eb5_2010.pdf
- Statutory Sick Pay (SSP) - http://www.hmrc.gov.uk/ebu/ssp-08-spec.pdf
- Statutory Maternity Pay (SMP) - http://www.hmrc.gov.uk/ebu/smp-06-spec.pdf
- Statutory Adoption Pay (SAP) - http://www.hmrc.gov.uk/ebu/sap-06-spec.pdf
- Statutory Paternity Pay (SPP) - http://www.hmrc.gov.uk/ebu/spp-bir-06-spec.pdf