Imperial Mortice Door Latch

[Fat Freddy's Cat]

The living room door latch has never worked since we bought this property so I took it out to replace it -

It seems to me I'm going to find a hard time replacing it as its dimensions dont match modern latches - I assume its imperial. Any advice where I might be able to get a replacement mortice lock/latch?

For info,

It measures (from front of faceplate),

Backset (to handle) is 130mm Full depth is 152mm to keyhole is 46mm

The height of the mechanism from top to bottom is 70mm

the faceplate itself is 120mm

[The Medway Handyman]

I bought some in Wickes last week. They had a choice of 2 1/2" or 3". So much for going metric in the 1970's!