Trees have extra-floral nectaries as well as some plants like field beans. They are supposedly for beneficial (to the plant) insects such as ants.
So perhaps this is what the bees were foraging? I know that honey bees find field beans difficult to access unless other bees have nibbled holes in the base of the flowers; but they do utilise the extra-floral nectaries.
By the way I was wrong about the brick red pollen which is (according to Margery’s pollen chart) dandelion.