An estate agent has forwarded me a contract.

It looks OK, apart from one clause:

"In the event of termination of this contract by the Seller, all outstanding marketing outlays incurred by the agents will be immediately payable along with a withdrawal fee of £150.00 plus vat."

Is this type of clause pretty standard, or is could it be construed as being excessive?

I'd say having a withdrawal fee is acceptable, after all if you withdraw from sale, they'd get £0 commission.However, if you feel £150 plus VAT is excessive, perhaps you could try to negotiate. Think you should also try to negotiate to have a some sort of fixed term, following which you can terminate your agreement in a much simpler (read cheaper) way.

Either you've misunderstood the question or I haven't made myself clear.

The situation is as follows: The agent gives unsatisfactory service, perhaps not enough prospective buyers coming to see the property. The Seller wants to terminate the estate agency contract, so that he can go elsewhere. The Seller is bound by the contract to pay the agent in order to get out of the contract.