Any guesses what lunch was?

Assuming that that is a female marsh harrier, which it certainly looks like - I would say that is a shellduck 

Great catch, both of the ladies involved ;)

I'm impressed she took something that big, must have been hungry!

Hi Cath

The few makings and general colour, plus the size relative to the harrier would suggest that it's a Shellduck 

I agree that would be quite a large meal to bring down herself, of course most raptors will take the easy route if possible.

The Shelduck could have been weak or even dead (carrion is always an option as it is easy to catch ;) )

Of course it could also be a trick of the light and the prey item is a Knot or similar which is not so large an option

Update.

Hi Cath

I was at the east end of the reserve today and saw the male Marsh Harrier, on the foreshore make an unsuccessful attempt at a small flock of Shelduck, roosting at high tide..

Looks like the Pair we have in the area believe in large meals rather than little and often

To be honest I would think that Shellduck are a bit big for the MH but I suppose the odd weaker bird would keep them well fed.

Thee M + female were reported together at the Goldcliff lagoons this morning giving every impression that they are a Pair, who knows we may yet have Marsh harriers Breeding  - Yay