Personally, i have been asking for cash for months now due to this credit crunch. If you feel uncomfortable in doing this, simply add £30 to £40 to the price that you want then reduce it to this price for cash. You get what you want and the custy think they are getting a bargain.
The thing is, you have done the work, then why shouldn't you get paid straight away. In the past i find that many people can't be bothered to go to the cash machine, but would rather write a cheque. My petrol bill in running my machine can be high if my earnings are high, so that's another reason for asking for cash.
Dave