No harm trying.

Get your lawyer to write in to the vendor's lawyer to seek an extension (reasonable extension) interest free. Simultaneously, get the agent to speak to the vendor regarding the same.

If agreeable, will be good to do up a simple variation letter to be signed by both parties as there will most likely be a clause in the SPA which states that there shall be no amendments or variations unless agreed and signed by both parties in writing.

Vendor's lawyers will also need to confirm with your financier on the new completion date and extended completion date.