I always assumed that alkathene was a generic name for the whole range of polyethylene pipe.
Generally speaking, in the UK, modern blue water-main is made from Medium Density Polyethylene (MDPE)
In the past black light density polyethylene (LDPE) was common, but it came in several different classes, so Class C LDPE required different fittings, or at least different inserts, to Class D LDPE.
The class was indicated by colour coded writing on the side of the pipe - as I recall they are pale green, dark green, sky blue and a sort of browny red - but there may be more in other areas. Several fittings manufacturers colour code their inserts to match the colour writing on the pipe.