I used the service offered by the MIGsofT website to network unlock my Orange SPV C500. It cost £20 and took about 10 minutes to complete. Dead easy and does exactly what it says. Recommended.
My plan was to keep my O2 PAYG sim and avoid migrating my number. O2 has lower GPRS and MMS costs and free SMS bundles. However, that plan seems to be foiled since O2 PAYG only offers browsing via WAP and dial-up rather than real GPRS. To be confirmed while I muck with all the configuration options. So far, I've got the basic O2 calling and SMS working fine.