Goff offered me the post-pay cheque option, but take a look at the invoice they left me: a £20 credit charge had been added, along with a note that "The credit charge may be deducted off the invoice total if paid within 3 days".
Not only is this ambiguous (may be deducted ??) but the credit charge has been added to the total amount to pay. I wrote my cheque for the amount without the credit charge added, ie, what was agreed on the phone, and sent it off to them the day after the delivery - on the Saturday of a bank holiday weekend, so they won't be handling the cheque until Tuesday at the earliest.
Keep your eye out for these hidden charges, which they seem to apply as an devious afterthought. If they want to charge for late payment, then they need to be up front about that during price negotiation.