Perhaps someone with greater legal knowledge would correct me, but my reading of the legislation is that 28 days is the maximum extension of the strike mandate, even if agreed by both parties. After that a new strike ballot must be held. A ballot on a new deal would not act as a strike ballot under the act.
234 Period after which ballot ceases to be effective.
(1)Subject to the following provisions, a ballot ceases to be effective for the purposes of section 233(3)(b) in relation to industrial action by members of a trade union at the end of the period, beginning with the date of the ballot—
(a)of four weeks, or
(b)of such longer duration not exceeding eight weeks as is agreed between the union and the members’ employer
Dave