Can I ask why you put down a 20% deposit? I understand that you were only seeking 80% finance but that doesn't mean you have to hand over the other 20% as a deposit - you simply contribute your share at settlement. I have bought three properties with deposits of $1000, $500 & $100 respectively. We also had another property held on a $500 deposit but subject to a building inspection which "failed" causing us to walk away, deposit intact.
Flatout, of course you can exchange contracts on a lower deposit than the "standard". However, if you as a purchaser fail to complete the contract (after exchange mind you - which I believe is what pompey11 is talking about in this situation since it is only 2 weeks to settlement and the contract has gone unconditional - after cooling off period expiry I assume) then you can still be sued for the difference in what you agreed to pay and the final selling price to a new purchaser (if it is lower). You can't just "walk away" after exchanging contracts.
Anyhows we are only arguing minor points around the edges, which is not a lot of help to pompey11 atm.
Trust it all works out pompey.
Aimy