As unfortunate as it sounds, my experience is it is best to serve every notice as soon as you are allowed to as per the Tenancies Act (including eviction notices).
If you let it slip, don't be surprised when you are left with 2-3 months of rent owing, a bond that doesn't cover 1/3rd of it (after you deduct cleaning and re-letting fees) and a phone call with your tenant insurance company about 'accidental vs malicious damage'. Happens every day.
I'm not some mega company that can afford stuff like that.
Some people just think of rent as an optional expense, to come after TVs and other junk.
I treat my tenants with full respect by fulfilling all of my obligations (and over and above my obligations - including immediate response to 'non-essential' repairs) and I demand the same in return.
Firm but fair - it's good business.
If they think it's a bad deal there are plenty others who will happily pay the rents on time.
In your case I would serve notices immediately, and get a PM with balls. No need to think anything further of it.
The only time I would consider otherwise is in the case of a good tenant I have a good relationship with (i.e. a long history of paying on time - which in this case you don't have).