Wow! I reckon they must have been starving! The 3 litres (or thereabouts) of syrup I gave them on Saturday was all gone by the time I looked this afternoon. Gone in about 2 days! There were a few dead bees in the bottom of the feeder - I guess drownings. Anyway I have added some more syrup (again about 3 litres), so I had better check this on Wednesday!
I scooped out as many bees as I could before putting on the inner cover. I think they can find their way past the inner cover so I hope I will not get too many casualties. If I do, I may have to come up with another feeding solution. A closed ziploc bag containing syrup may be worth a try. Of course I will pierce the bag so that the syrup can be reached by the bees.
Anyway I hope it is the bees eating the syrup - either that or the feeder is leaking. I did look but couldn't see any leakage at the bottom of the hive. I'm guessing that if they are this hungry I may need to get some protein in them as well.