all taps and showers in the house. But as far as I can see the mains
pipe coming in under the kitchen sink is just 15 mm. I need 22 mm.

First actually test the flow rate. Under some conditions, 15mm can actually
give an adequete flow rate. If not, then the replacement will not be cheap.
Think in terms approaching four figures, especially if the route goes under
block paved driveways etc. You may be forced to have a water meter. However,
unless you use hosepipes and sprinklers, meters are likely to save you
money.

Also, you might want to check that the mains in your area are good enough.
In some cases, ancient Victorian mains are well undersized as requirements
and housing density has increased, and will have poor flow/pressure
characteristics themselves.

Christian.