I would agree with Trevor, originally most of my commercial work was agreed on verbally.
Now I get all contracts in writing as I have been 'caught' out by a clever loophole from a management company/residents association. The contract was 6 months behind on payments (after 4 years of good, if occasionally late payment), when the residents decided that they were going to switch management company, this left myself and the cleaner and the gardener unpaid.
The management insisted that the clients were responsible not themselves as the agreement had been made on their behalf, the clients turned round and said that the management were responsible for settling all invoices. The debate went back and forward for a year. In the end I got signed statements from all residents bar one, saying that they had paid their fees in full. Because there was no written agreement between either myself and the management or the residents and I had not managed to track down one of the (now living elsewhere) residents to confirm their payment status, I could not proceed to the small claims court.
Lesson I learnt-get it in writing,because everything is fine until there is a dispute.