John Borman wrote:

So should I be worried that the guy that's installed central heating into a
renovated house cut a U section in the top of the beam ,of 1st floor.


That depends.

If its then had e.g chipboard nailed on top, a lot of the strength and
stiffness is restored.

Note that the building regs tend to look at maximum defelection under
load, as ultimately failure load is way higher. You could have maybe 6"
of sag on a beam spanning 5 feet before it broke, but this is
unacceptable in a floor :-)