The owner has been advised by the builder that a 15mm supply will be
adequate,

As a general rule, I've only used 15mm for basins, toilets and appliances.
Showers and baths get 22mm. However, you appear to be installing a combi
boiler. If the mains pressure really is low, you should check that it is
still enough for a mains pressure installation. What sort of flow rate can
be supplied?

Christian.