He’s found it

The lady in the pet food shop where we went to buy dried mealworms mentioned that ‘ her ‘ robin seemed to enjoy sunflower hearts too, so we bought some.
Yesterday I saw a sunflower hearts feeder in a bargain shop, so bought it for a bargain price.
Today Mr. Beewatcher filled it up, respecting the instruction not to overfill it, and hung it up.
Then - well - see above :-)