Okay... now that the boring paranoia stuff is out of the way.
Since you are in the UK your material falls under the fancy automagic copyright laws most western countries have agreed to. Those essentially state as soon as you put something you have created into some form of physical format you automatically own the copyright to it.
So if you think of an original poem or melody as soon as you write it down on paper it is automatically copyrighted and you own all rights to it.... technically.
In the digital age this translate to digital media as well like hard drives, writeable discs, uploading to a website, etc.
While this is the case it is easily manipulated by nefarious neer do wells. Essentially you have to PROVE in court that YOU are the original creator.
This is why people used to say "mail yourself a copy and don't open it". That way there is a postmark. That's old school and isn't the best way to do it but it's still pretty good any judge that isn't totally corrupt will see that as convincing evidence that someone is trying to screw you... unless of course it turns into an "I already wrote that melody BEFORE this guy did" and they can prove it using the same types of methods.
This is why people shell out money to actually pay to copyright stuff through their government bureaucracies. This is not very practical though because here in Canada it's $60 freaking dollars to copyright a single work. Even then it can still be disputed but if you get the copyright that ends up looking better in court (and it makes it easier for the lawyers/judges to sort out).
What I've been trying to do myself, since I don't want to pay $60 every time I fart into a microphone, is use "burden of proof" type methods. Things like burning files to disc with the original file timestamps (so when you open it you can see the date the file was created). I signed up for SOCAN which is (one of) the royalty collection agency in Canada who don't deal with copyright but you can register your songs with them for royalty collection which of course provides some proof of when the material was created (well when it was registered anyway). Sites like soundcloud or other uploaders will put a time stamp on when the file was first uploaded. Linking to it on a forum like this adds to prove a date of when you produced the material too.
So, if it comes down to it, you just bury the court with all this and if the aggressors claim is thin then, again if the judge isn't corrupt, you should be able to shut it down.
HOWEVER, if the other party has proof that PRECEDES your proof (which is impossible if the dispute is indeed over something you created) you may be out of luck.
Where it gets murky are these intellectual property trolls who patent/copyright things in such vague ways that it covers the scope of all sorts of things people are LIKELY to put out. Then they strike. Usually what happens is they'll issue some kind of letter/warning demanding compensation to avoid a lawsuit.
So it's basically a type of extortion. You don't want to have to pay lawyers and go to court to fight something worth far less than the lawyers costs and the "trolls" make sure their monetary demands are less than what it would cost you to go to court as well. If it DOES go to court and you win (which is likely) you are still out time and money.
Thing is these people don't care whether they win or lose or if you even pay attention to them. They just put this crap out there and if one out of ten people get freaked out enough to pay out then their bullpizz is worth it. They are scum, it is a racket and most sickening of all it is perfectly legal.
Not saying that is what is happening here BUT if that's what's up do some research on this behavior. People have been pushing back against these scummos more and more which is really what is needed to change things.
Again... I am not a lawyer and I am not YOUR lawyer. If this turns into a big thing you really should contact an attorney that specializes in copyright law or simply call your local copyright/patent offic over there in the UK. They can tell you everything you need to know.
Cheeeeers... and don't let me make you paranoid. Just want you to be armed with knowledge. Most powerful weapon in this age.