No chance that you are nailing into a steel lintel under the screed,
I suppose? That would do this every time. There must be some sort of
material change for this to happen at a consistent depth - I'd do an
investigatory drill to find out.

R.

Thanks guys,

Took a stroll around B&Q today, and in the electrical section along with the
cable clips, they had the tower pin plugs. Job done. In answer to the above,
there was nothing behind, just rock hard cement I guess, but drill went in
ok.