I precisely measured my old lock and used the GB DIY internet guide to order a replacement. The replacement arrived very quickly. Although the principal dimensions of the lock I ordered were correct, the internals of the lock are incompatible with the locking mechanism. My requirements are for a gear-driven drop bolt which at the same time drives out the mortice. The replacement lock supplied has a separate RISING bolt rail driven by upward movement of the door handle, and the mortice is driven separately by the key. Nowhere on the GB DIY webpage for the principal dimensions measured does it state that there are different mechanisms available within the envelope dimensions. DIY's "Nathan" has told me that I've ordered the wrong lock. Because I've opened it to determine the differences in the internal mechanisms and photograph them for GB DIY, they won't accept returns. "Nathan" says that I have to send a photograph of my old lock, and he'll identify the correct lock, and I have to buy another. I've previously rated GB DIY highly, but this has to be a zero. The lock identifier page does not warn that there are different mechanisms within the described principal dimensioned lock drawings. I won't be buying any other item from GB DIY.