Still not had a reply outta them, but tbh the tank cap looks pretty bad from the photos.
As it happens, after much 'fiddling', swearing and knuckle scrubbing I finally managed to pick the seat lock myself with a couple of metal strips outta car wiper blades n a gallon of WD40! (A handy tip to all the thievin pikey chav-types out there!;)
Still need to replace the barrel tho as its only a matter of time before it fully fails...
Ive also had the tank cap off and apart and treated it to a proper clean and WD40 bath. Its freed the action a bit so it should be good fer a little while yet...
Still on the lookout for a complete lock-set fer the Falco tho, just in case...
Cheers fer the advice/help guys n gals!

Two is the magic number... ;)