I'm in a somewhat similar position. I have tenants that are WAY under market. They take wonderful care of the property and perform all general maintenance. I inherited them when I purchased the property in January. They've been in the house for three years, and they rented their previous home for 25 years.
Their payments come in just after the first of the month. I've explained the terms of the least and my expectations of payment, yet they're still a bit cavalier in their payments.
They're TAW, and I've decided to move on from them and issue a notice to quit. While I understand the value of long term tenants and prideful renters, I don't run a charity. My mortgage is expected to be paid on time, so I expect my rent to be paid on time.
It sounds like your situation is not going to get any better due to the medical condition you describe. I would suggest moving on from them.
