Sorry to hear you're having such trouble with the squirrels, Hugo. My sister and brother-in-law used to have similar problems but (as I wasn't providing the food) it used be fascinating watching the squirrels working out how to beat the latest "squirrel-proof feeder.
I don't have a problem with squirrels but have been trying some squirrel-proof feeders to deter big birds and give the little ones a chance. They've stopped the jackdaws but the magpies with their longer beaks can still reach through the outer cage and can empty a suet ball or nut feeder in a day. For the moment I've given up on those, which leaves the little birds with only the seed feeder. As it's rather windy here, the feeders are all fixed securely to a wire fence, which probably helps the magpies.