Name of old scifi movie?

I cant remember the name of this movie.... i believe it was in london, workers found a spaceship underground. Eventually after getting into the spaceship they find a couple of dead aliens. Somehow the 'thought matter' of the aliens creates a electric type large monster. they kill it by putting some type of construction equipment into it and grounding it? i know this is vague but its all i can really remember. anyone know the name of it???