If either the buyer or seller are late in completing, interest is payable to the aggrieved party at the contractual rate (the rate specified on page one of the contract). Compensation and other costs resulting from late completion ( e.g. removal costs, bridging finance) may also be payable, depending on the conditions in the contract.
If the buyer fails to complete, they forfeit their deposit of 10% of the purchase price. If a 10% deposit was not paid on exchange of contracts then the seller may pursue them for the balance of the deposit and can also sue for breach of contract.
The special conditions in the contract must be checked carefully when the contract is being reviewed by the conveyancer to ensure any costs payable due to late completion are not excessive. The contractual interest rate should also be checked to ensure it is reasonable.