If the clause says purchaser, then just the purchaser. But I would anticipate there will be another clause which will allow the vendor to get out. It's a common thing for off the plan contracts where pesky things like council approvals can ruin everything.
Let's just say there is no other clause that allows the vendor to get out. Would you pursue the vendor for breach of contract, when clause 4.3 says only the purchaser can end the contract? Or would you pursue clause 4.4 which says:
'(“Extension Event”) that in each case the Sunset Date will be extended by such period as the Vendor’s Solicitor certifies as a reasonable length of time for the delay caused to the Development. The decision of the Vendor’s Solicitor will be final and binding on all parties. The Vendor’s Solicitor may extend the relevant period by certifying as to a reasonable length of time for the delay caused to the Development on more than one occasion. The total of all extension claims must not be greater than 6 months from the Sunset Date.'
Knowing full well that this is a staged development & that the vendor can change the Stage the unit falls into at any time, in order to push it beyond the 6 months allowed for in the contract?