Now that the engine is in the chassis, I can get under it. I seem to have a bolt missing. You can see the hole at the bottom of this pic...
Pic Here

I assume that there should be something in there??

Ta

Nope, I think its the water pump breather hole but looks slightly different to my 5PW so not sure.

On mine anyway, there's a little copper pipe that has an O ring on each end that exits out through the sump that allows the (internally mounted) water pump to breathe. Bit of an odd design venting out the bottom of a sump but as long as the O rings are OK, there's no leaks.

It's definitely the water pump drain hole. All becomes clear when you drop the sump. BTW I did have a bolt missing when I refitted my sump. Looked high & low for it 'cos I definitely had it. Eventually dropped the sump again & there it was sat in the bottom of it! doh!