Eh? I'm no expert but before I started flying I used to employ a small number of people, and therefore had a working understanding of UK employment law. Briefly, an employment contract, as far as I understand, cannot be entered into "retrospectively", as you put it, without the agreement of both parties ie. it would be a new contract, superceding the old one with both parties agreeing to the changes.
You do not state whether you already have a contract, and I assume you have already done the training on the aircraft in question, later (by 12 months) to be asked to sign a bond.