I am not aware of any "legal time certificate". But for work of this nature 12 months seems ridiculously low. I assume your insurer will continue to insure the house and they should have a close interest in making certain that the work is done properly and will last a long time - otherwise they could have to pay out on another claim fairly soon.
You need to talk to them.
I also assume a qualified surveyor or structural engineer (appointed by your insurer and paid by them) has specified in detail what work is to be done, and that they will oversee the carrying out of the work. This is what should happen. If such people (who should be entirely independent of the firm employed to do the work) have not been involved then I would say the whole process is questionable & gives the impression your insurer is trying to do the job on the cheap.
If a surveyor or engineer has been involved you could talk to them about the guarantee period. If none have been involved then query this with the insurer & - if necessary - make a formal complaint and go through the procedure to the Ombudsman.
I am puzzled about you needing to "sign off on the works schedule". This gives the impression that the insurer is placing responsibility on you to confirm the accuracy and rightness of the work to be done. I do not see how any layman can be expected to do this with work which - if it is to be done properly - must involve considerable technical knowledge. Signing off on the schedule is something the surveyor or engineer is much more qualified to do.