On the occasions I've gone out and bought the 'fat balls' and placed them in it, inevitably they get a little bit pecked at, but not much. Pretty soon, they go moldy. It seems to me that you need to hang them out individually, unless you have birds which are particularly keen.
Now, I occasionally make 'fat cake' for our birds, and hang it in upside down yoghurt pots, etc. They do seem to like it. So I was struck by an idea. I got an empty kitchen roll tube and made up some mix (no real recipe, I'm afraid, seed/oats, peanuts crushed in the food processor, shredded cheese, little bit of suet, enough lard to make it harden okay...). I pressed it into the cardboard tube, really firmly so i got as much as I could inside. Then I wrapped it in cling film and put it in the fridge:
About 36 hours later, I removed it from the fridge and sliced it open with a knife.
Got my feeder, and bingo! Perfect fit.
The most recent photo shows it like this:
Although shortly afterwards it skewed, and Andy pronounced it 'structurally unsound!' Today it is in two halves, in the bottom of the feeder.