Pension Opt out Payments

Dear All, I am writing this on behalf of 13 Marie Curie Fellows based in UK. Most of us opted out from the pension schemes of our host University and we did this within 2 years of contract. Therefore we were able to get our contributions back. However we don't know what happened to Employer's Pension contribution when we opted out. What is your knowledge on that? Can you please me point to any up to date documents that would include regulations on that? Thank you very much Pinar