Thanks for you reply. First of all, I'd like to say that we are all small independent coders who should support each other, so I don't want to get into a dispute, and it makes no sense for us to confuse the public by having a game with the same name - noone benefits from this, some people may end up buying the wrong game on both sides, so naturally I hope we can find a friendly solution that will help to avoid this.
To answer your other question, I understand that according to EU law, a registration is required for a trademark . However, in both the US and the UK, common law trademark rights are based on a 'first to use' basis. This means that the rights to use the name in those territories are automatically given to those that can show they used it first. Since the game was launched in 1983 and is quite well known, it is clear that common law would apply here, which is why the original author retains the name as a trademark.
But, as I said, speaking from one coder to another, I'm sure this is mostly a misunderstanding so hopefully it can be settled without further dispute. I sent you a message on twitter if you would like to discuss it further in private.
Also, good luck with your (renamed) game, and best wishes for your future projects too.