Ian White wrote:
Rusting and swelling of old iron downpipe fixing spikes is a known cause
of cracked render, and our house is a perfect example. But when removing
these spikes, they tend to snap off in the bottoms of the holes. Is it
OK to fill over the deeply buried bits... or is that storing up trouble
for later?

Theoretically, as the holdfast (or whatever) rusts, it will increase
in size with an enormous force being created (to about 10 times as
big as it was, IIRC). IRL I shouldn't think it will create a great
problem, and you can always get them out anyway.