Ann Minogue writes (1 August, page 43) that the new JCT major projects form does not give extensions of time for strikes.
That is indeed what the guidance note to the form says, but it is not correct. Because the form gives extensions for force majeure, and because force majeure includes strikes (Hackney Borough Council vs Doré), the form does in fact give extensions of time for strikes through the back door.

Postscript

Nicholas Lane, Winward Fearon, via email.