I recently had a kitchen extension built but I'm having problems with
the hot and cold water pressure. These were both ok before the
extension.

There are, I believe, 2 possible reasons for this:

1) The mixer tap: I can't now remember the specification for the tap
but it does have two very small flexible hoses that I'm concerned are
affecting the flow.

2) The sink only moved about 2 meters but the plumber used flexible
hose instead of copper embedded in the concrete kitchen floor. I'm
concerned that these hoses may have become partly crushed.

Which do you think is the most likely cause?