bilge wrote:
I have been told that the 15 mm gas supply pipe to my 8 year old
Vaillant TurboMax vuw 242E should be 20 mm .

Is this correct?

The installation manual only states a minimum of 15 mm.

The rule of thumb is 22mm from the meter most of the way to the boiler,
with up to 1 metre of 15mm to the boiler itself if necessary.
However thumbs are all different sizes :-)

If the run from the meter to the boiler is very long (over 20 metres)
and/or the boiler is very big (over 28kW) some of the pipework may need
to be in 28mm. However the maximum permissible lengths for given flow
rates depend on something like the 4th power (i.e. squared and squared
again) of the pipe size, so 3m of 15mm pipe is equivalent to perhaps
100m or so of 28mm; in other words even a fairly modest length of 15mm
will impose an unacceptable pressure drop in the pipework.