Exploitative and heavy handed

Savvy plumbing was called out on a weekend to help unblock toilet drains. They spent several hours trying unsuccessfully to do this, and returned the next day with a high pressure hose. I expected the bill to be hefty because of the weekend callout, which it was and I paid it immediately. After paying the amount, I received a call from their office, informing me that I had only paid half the account. I was informed that an amended account had been sent (which at that point, it hadn't). The accounts department had made an error and failed to bill me double rates for the weekend callout. The representative was confrontational and completely unapologetic about their error. Only after having put down the phone, did the second email come through, with an amount that was almost double to the already hefty bill I had been charged. (A different independent company gave me a quote for almost 50% less of the total). When I called back to try to talk to a manager about how the incident had been handled, I was told someone would get back to me. No-one did. I received a follow up email, demanding payment, which did not reflect the amount I had already paid. When I pointed this out and asked again for the person in charge to be in touch, I was told that 'he has management to sort it out'. Still, no-one called. Less then 2 weeks after the second invoice was sent, I received a letter of demand from a debt collection company. Talk about heavy-handed. This company has a lot to learn about client services, conflict resolution and not exploiting your customers.