Most recently it was this Home Safe from the long defunct Derby Savings Bank. This little safe belonged to my grandmother. It's metal, and even empty it weighs almost a pound (that's around 450 grams if you're reading this in metric). One side has a slot for coins and the other has a round hole for notes.
When I went round Birmingham's Back-to-Back houses, there was a similar safe on the mantelpiece of the 1930s themed house, so I'm guessing my grandmother's safe is from the same era. I am told that the bank kept the key to the safe to prevent savings being raided for the gas meter. You would have to take your safe along to the bank for them to open it and pay the contents into your account. Unfortunately I don't have the savings book that would have accompanied the safe so I don't know how much my grandmother managed to squirrel away in there. I'd love to know what she was saving for.
Apparently safes were manufactured for lots of banks across the country, and also for the Post Office. Do let me know if you've got one like it.