As title, many thanks.

Depends on what you need but there is no specific specialist at the moment catering for Montesas here in the UK so you may struggle for engine parts. You can try Millers, Gordon Farley but unlikely they'll have much. Dave Renham via In-Motion is starting to stock parts for other marques as well as Bultaco so try there too. Sandifords are no longer interested in the old stuff (so I'm told) and have sold most/all of their spares stock to Jeff Nolen in the US who trades under Vintagedirtbikes.com. There is also Southwest Montesa in the US.

In Spain there is Ivan Cirre who has a lot of parts but you can't order with paypal or credit card of his 'net shop, you have to send a draft or bank transfer (or at least you did) so it is a bit awkward.

Try Ellastone Offroad or Steve Goode for used parts and there are two sellers on ebay - Classic Parts and Trickytrials - who regularly put used Mont parts up for auction (some on there now)