"NetMax" wrote
It's also been observed that purchased mature Guppies
have a much shorter life expectancy than fry born in your tank. The
transition from brackish to freshwater could be a contributor to this as
well.

These observations are on N.A. fishes, so if U.K. fish go through the
same thing, then I'd recommend stocking your tank once, using a high
female/male ratio, and provide a tremendous amount of cover/shelter for
the fry. The plan is to 'milk' the purchased Guppies for as many fry as
possible before they die off. Be very prudent about removing dead &
dying mature Guppies (weakened fish become hosts for a variety of
contagions whose ability to infect other fish can increase when
ingested).

I can confirm that in this part of the UK at least,
tank born Guppies are much healthier than their
shop bought parents.
Mind you, that wouldn't be hard.

--
Graham Ramsay
Blairgowrie (UK)