Review of all 4 parts:
Meglos is a very routine, unexceptional story with some rather bizarre and silly aspects but it is not really bad in my opinion just a bit below par for the high standards of the show.
The writing is okay but the way it is presented on screen is a bit off. It is a jarring mix of serious and silly. A villain in the form of a giant cactus was an unnecessary daft idea. Why could it not be a spiky plant -like creature with some form of limbs and features, perhaps a different colour and certainly not a standard large green cactus in a plant pot! It was way too literal. The mercenaries have a decently acted leader and reasonable premise but while the leader is mostly serious his sidekick is a ridiculous, gurning pantomime character and the costumes are terrible. Them wandering round in circles foolishly following Romana is also silly.
On the plus side we get Jacqueline Hill in a guest role as Lexa. She had wonderfully played Barbara, one of the companions from the very start of the show in 1963 and it is nice to see her. She is perfect in the role and although it gives little chance for her to shine she is a great addition. There are also some interesting and menacing scenes and fun make-up with Tom Baker and another actor both transforming into cactus people. The spiny, bumpy green skin is quite good and the evil doppelganger gives Tom some different stuff to do.
Overall a bit disappointing but not terrible.
My ratings: Parts 1 to 4 all 6.5/10.