missed rent again

Rent Arrears

I have tenants in one of my properties who has again missed their rent payment. This has been happening since june of this year. When I chase them they come up with the cash but it's always late. Its gone from the 1st of the month to the 28th and still this month its late! The problem is in all other aspects they are good tenants. They really do treat their home as if it's their own purchased property and because they are tradesmen they have spent a lot of time and own money on the property and have greatly improved it. Great for me and I haven't asked them to, their choosing but i'm not happy to let the rent fall by the wayside because of this. Would anyone else serve notice or just start more formal tactics (ie formal letters rather than phone calls) to remind them of their responsibility to pay on time?