Thanks for the tip. Because its hollow over where the hottest area is,
when the heating is on, I hope that its the sand that was surrounding the
pipes. Could it just be localized damage?

Heating pipe under concrete? I suppose it could be in sand. If I were really
*forced* to concrete-in a heating pipe then I would have used plastic pipe
and pipe insulation. I suppose it might sound hollow if a long-term leak has
washed away the sub-soil. You might be lucky and just have localised damage
but there will be a risk of corrosion elsewhere.(That said, I have a copper
rising main plastered into a damp wall which hasn't failed yet, but it is on
my "must fix" list)