What the arrangement actually is
In a converted house with two or three flats, the leaseholders commonly own the freehold between them, usually through a company. There are still leases and the leases still provide for a service charge. What is absent is the practice of collecting one: instead, work is done when it needs doing and the cost is split at the time.
That is not an irregularity. It is the sensible answer for a small building with predictable, infrequent expenditure, and it is why so many of these arrangements exist.