Iam looking for a film title

[Mozza63]

My wife keeps asking me to try and find a British film that she remembers watching as a child back in the 70's.
She has no idea who was in it. All she can remember is that it is about a pyramid of gold bars, that is in a London bank vault, and is in constant view, and is manned by guards. So, the robbers go through the sewers, and dig upwards through the floor, and remove the gold bars from within the middle of the pyramid. So, that from the vault it looks like it is still intact.
Does anyone remember this film?
Thanks in advance.

[CaptainWaggett]

Loophole?

[Mozza63]

It's not Loophole, early seventies.