Valid trustee appointment?

Settlor signed Trust Deed which declared a trust over his property. A restriction was added effectively stating that there should be no disposition without consent from trustees.

The co-trustee did not sign the original trust deed and the Settlor has now died. Can the co-trustee sign now and take up his appointment, or should it be the representatives of the deceased who now act as trustees?

My thoughts are that the co-trustee can sign and act as trustee as the deed is valid from the time of signing, and the co-trustees appointment is valid when he signs?

Haroon Rashid
I Will Solicitors Ltd