Numrich Gun Parts has the magazine in stock. They will ship to foreign countries and Australia does not have restrictions on them.
On the top of the page it says you are from Australia.
There is only 1 parts supplier that I know of in the UK. It is MidwayUK.com. I have searched their site and they do not have one. They may be able to order or find one for you. http://www.midwayuk.com/ I have searched the internet and can not find gun parts dealers that stock magazines except for Enfields and BSA's.
There are not parts suppliers in the UK that specialize in parts for out of production guns. No one stocks the parts or makes them for older guns over there. The Model 340 has not been made in years. We have a huge choice of aftermarket parts over here and parts for older guns are being produced every day and easy to find.