I might have a spare Karrimor buckle.
It won't be exactly like the one in your picture but it is a Karrimor branded one. Both male and female parts.
What size webbing is it - 20mm or 25mm?
Don't build your hopes up, it might already be on something. But I was in the shed a week or so ago and I had a bag of buckles and bits and I'm pretty sure I saw a Karrimor one in there.
I won't be able to check until tomorrow. But if it is there, and if you want it, then it's yours.
Let me know if it needs to be exactly like the one in your picture (my one won't be).