They're had 18 months - the Council have another tenant waiting - and they've already had a major extension. See Martin's post. No plan B, just tryingto bounce the Council into another extension.
Reading between the lines the Museum seems to have wanted "support" from the Council to help them finance a move - this was never going to happen.
"The Centre has had since October 2021 to finalise their plans to leave the site. In order to assist the centre, the Council extended their lease by a further 12 months in 2022, to the end of March 2023, for that purpose."