It appears they're mostly gone already. However, most likely you are over-watering.

Thanks for your response.

You are probably right about over-watering being the cause but if that's the case I'm not sure why the other plant isn't the same. It could be because it is in a bigger pot with more soil. Fortunately where I live we are experiencing a warm spell so hopefully the plant will be able to recover faster. How often should I water?

Regarding the other plant what can I do about the holes in the leaves? Should I be using some kind of insecticide?

A little bit of googling goes a long way (reposted from search):

Holes in the leaves

This is usually caused by caterpillars – there is a moth that can appear about August or September that will chew the leaves of the zonals which needs catching in the evenings or eradicating with a systemic insecticide. Geraniums are rarely affected by slugs and snails.

As to watering, I'm no good at telling you in a way that will probably mean anything..but they should dry out significantly before watering. Not to the point of wilting, but near that.

OK, thanks. I will give it a try.