In theory you could use the email address from your customers GoCardless Direct Debit, import that into MailChimp and then maybe once a month put out an offer for an Addon service to your existing customers. Such as 10% off Gutter Clearing (to get their attention etc).
That's what alot of the US guys do when it comes to marketing to existing customers. I've never done this but i have thought about it.
I know that you don't use GoCardless but maybe the direct debit provider you use captures email addresses?