Hi, Help on Peugeot Squab Engine Number

Hi, I've recently imported a 1997 Peugeot Squab 50cc, that I've had from new, out in France. I now want to register it in England, but need the engine number. Can't find it anywhere!! It didn't come with a V5 type document so no help there.
I know the newer version of this is the Trekker, if anyone has one.
Any ideas gratefully received.
Thanks
Nick