Hi thanks for your reply, sorry for any confusion.
The drain plugs / bungs were from the rear hatch back door,
(the tape over the drain hole photos were from the front doors - no issue with water for the front doors, yet.)
In terms of dealer checking, the nearest MG dealer Arnold Clark has said they don't have any appointments until June, which seems ridiculous to me... so not really an option unfortunately.
So far, having removed the drain holes has sorted the immediate problem of water sloshing about inside the hatch door, however... it still doesn't answer the question of where it's coming from or why it should be in there in the first place... Hopefully MG will help, I'm slightly disappointed at the total lack of responsibility MG has so far taken on this. (from the dealer I bought it from and from the the local dealers).