Tosh,
This may sound like a silly question, when the customer has filled in the SO form and handed it back, do you send it to their bank or yours??
It goes to THEIR bank, not yours. It's basically an instruction by your customer to their bank to credit your account a set amount each month.
I may fill out the form with the customer and send it off myself (to their bank); just for my own peace of mind. I know it's done, but you could leave it with the customer to do; just fill the bits in relevant to yourself.
To 'sell' it to your customer (in this day of banking fraud), you can also tell them that it's NOT the same as a Direct Debit. You have NO control over increasing the amount taken (unlike a Direct Debit - which you need to fulfill a certain criteria to be able to use). You do not get anymore of their bank details that what you would have if they paid by cheque.
Give 'em a bit more waffle along the lines of, 'I'll receive payment for each month on the first, and then clean your windows sometime during that month. I'll leave a note, saying I've been so you can check your windows and mark off my clean on your calender. That way, you can keep track and we're both straight with each other'.
When filling out the form, unless the customer has an excellent memory, ask them to bring their cheque book. The sort code, account number, account name and (usually) the bank's address will be on the cheque.
The customer can cancell at anytime, usually in writting; but also by telephone banking; as long as THEY give their bank four days notice.
Try it with one customer and see how it goes.