Delayed Completion date

Rambosmum

Once you've figured out what you want to to do and what risks you are prepared to take, you say to the estate agent "our original offer was based on x, which is no longer the case and we want to revise our offer to xxx"

The estate agent will pass it to the vendor, who will agree or not. The vendor may withdraw, they may simply say no, or they may agree. You need to be prepared that you could lose the house, even if you revert back to your original offer. 

Octothorpe

I'd be pretty peeved if my vendor wanted a 5 month grace period! What's to stop it all falling through?? Or in 4 months they just decide not to move?

My advice would be to walk away, not renegotiate.