Thanks for the advice, I'll give it a try.

I will be using pin 10 to power a MSF receiver which takes spurious gulps of
current (less than 5mA), hence the capacitor. I will look at the spec for
the receiver to find out if I really need a cap at all. The code only
changes the state of the pin once every 1 minute (at the most); thats how I
spotted that the pin was remaining at a high level.

I see what you mean about the read-modify-write instructions and how they
can unexpectedly change the state of the pin especially if the pin is
connected to a capacitor.

Thanks again
Alan