Unknown airdropped munition

[TimG]

The attached photographs are of an item recovered from a reservoir near Heathrow airport. The location was apparently used during WW2 for testing various air dropped munitions. This item was concrete filled. I cant assist with dimensions as the photos were taken almost 25 years ago. However, the box object propped up against it in the last photo, is a packet of Polaroid 552 film (5 x 4).

Any suggestions as to identity most appreciated.

Thanks

TimG