Inevitably, it's the Pontipines.
Derek Jacobi always introduces the Pontipines thus:
Although they're only small
And even when there's 10 of them
They're hardly there at all
[My emphasis and existential horror]
In fact, aside from angstrom goats, the Pontipines are the smallest known living organisms:
They are so small, they can only be filmed using an electron microscope:
A runner prepares a slide of Pontipines for a scene with Makka Pakka.
As evidenced by their unusual costumes, headgear, and reckless number of offspring, the Pontipines are members of the New Birtzentruber sect of Pentecostal Christians. They are forbidden from using any technology invented after 1607,* which is why whenever the large electric bell attached to their house goes off they perform a ritual dance of cleansing.
Next door to the Pontipines live the Wottingers:
Equally tiny, the Wottingers belong to the Old Birtzentruber sect of Pentecostal Christians, forbidden from using any technology invented before 1607.** Most episodes featuring both Pontipines and Wottingers focus on extremely tetchy and detailed arguments on the precise Biblical definition of a lawnmower, and when Mr Wottinger will get his back.
*With the exception of Mrs Pontipine's binoculars, a gift from Bill Oddie.
**With the exception of clothes, a gift from Bill Oddie.
Read part 1 - Igglepiggle
Read part 2 - Makka Pakka and Upsy Daisy
Read part 3 - the Tombliboos
Read part 4 - the Ninky Nonk and the Pinky Ponk
Read part 5 - Jacobi's special list
Read part 7 - Shunka Warak'in