Yes I lost one once that way ( not on the Lancia )
I should post a photo of the one I have and see if anyone recognises it. I've a feeling its for a Coupé ( perhaps an earlier series )
Thanks SanRemo for looking in your Spares department.

You're right it is an 'HPE' filler cap. And yes I did notice the faint impression of the Lancia shield! A nice detail.
If anyone else happens to find a spare one do drop me a line.
I noticed that there are plenty of aftermarket caps available online, that I'm sure would work fine, but I don't like the look of them.
I'm quite fussy with things like that.
I'll check with Omicron also, I recently ordered a rubber seal from them to replace the worn one that seals the rear door ( kindly suggested by Nigel ) to seal off exhaust fumes entering the cabin. I improvised some small foam parts to fit in the recesses on the worn seal and it worked. So I'm looking forward to doing the job properly.
She's running well by the way, always such a particular joy.
Thanks everyone.
Regards
Andrew