It is interesting to note that almost all the plastic plumbing sceptices openly admit that they have no direct experience of the stuff. I have every variant of Hep20 made on my boat , starting with the original brown Acorn plumbing (which was bought out by Hepworths years ago) and have never had a leak despite much of it being more than 25 years old. I can recommend Hep20 without hesitation.
I have been dioing my own plumbing both at home and on the boat for more than 30 years, and in my experience plumbing leaks are invariably down to poor workmanship, not poor components. it is even perfectly possible to make watertight joints with cheap Chineese fitting so quality products like Hepworth and the newer style Speedfit is a piece of cake.
On the issue of rats and mice, Catweasel is correct about them eating it, which is why Hepworths specificly state that Hep20 should not be used in cellars where there is a risk of rodent attack. It is also advisable to use copper pipe within 12" of any heat source such as a boiler or fire.
Yes Alan five types. Types 1,2,&3 although cosmetically different are compatible with each other, types 4 & 5 are compatible with each other but not with types 1,2,&3. Types 4 &5 can be distinguished from there earlier versions by the raised end to the lock caps whuich are flat on the earlier types.