“8. TRUST FUND SHORTFALL [SHPL] jointly and severally covenant with the Trustees to add sufficient further funds to the Trust Fund to make good any shortfall in the event that the Appointed Actuary certifies that there are insufficient funds to satisfy the trusts set out in clause 4 and such addition shall be made within 12 months of the Appointed Actuary's certificate or within such other period as may be agreed by [SHPL] and the Trustees.”