I seem to need to bleed one of my radiators quite frequently, every 10 days or so. Any idea why? No obvious puddles anywhere!

I am not a plumber. But i have been told that air will work its way to the highest rad. If its possibly the highest that might be why and its moving from the lower rads.
Just to repeat i am not a plumber!

I think you might be right about it being the highest radiator (even though it appears to be the same as all the others on the top floor), still not sure why I need to bleed it so often though

generally caused by corrosion process going on in your system causing "gassing up"(flammable) get it flushed out this will cure .

We had a problem a couple of years ago where the water pipe from the "top tank" in the loft was blocked (it was a gravity central heating system) which meant that air bubbles persisted despite bleeding. In fact excessive use of the bleed key worsened the problem as it allowed air into the system but water wasn't able to replace it.

Major PITA that needed flushing of the system by Brit Gas (sadly, robbing b@stards, sorry if anyone works for them).