Slag, the defence the US calls the "defence of parody" is unknown in the UK. We simply judge whether there has been a substantial reproduction of the original.

George, you don't become co-owner.

FIrst in order to reproduce (or do any acts restricted by copyright) in relation to a protected work, you need permission. If you can't get it via the PRS/MCPS then you have to get it from the copyright owner or appropriate representative.

With that permission, you then produce a work that both substantially reproduces the original and also differs from it. You derive (if there is work of sufficient substance) copyright in your work. When the combination of the two is performed or reproduced, it results in the doing of the same restricted acts to two sets of works - the original(s) and yours.   

When you get the permission to make your adaptation, the rightsowner of the original will usually make it a condition of the necessary permission that you pay him a proportion of what you charge for the use of the resulting adapted work, or what you receive from it (not necessarily the same thing).