How can I fix a bristan mixer tap leaking around the spout?

I have what I think is either a Bristan Manhattan or Montreal mixer tap in my kitchen. It has developed a leak around the spout but I can't find any o rings to replace the worn ones. Can anyone tell me how I can sort this out? I have tried wrapping PTFE tape around it and using silicone grease but it is still leaking. I don't want to have to replace the tap for the sake of two worn o rings.