There's not much info on anything made by "RDM Creations" to be found (I did see that Ideal World still sells them for 250 GBP)
but you can still try the usual approach:
Disassemble the quad enough to access the main board inside then switch the connections for the motors.
If the previously dead motor works (and a different motor quits) then you know it's either the section of the board
that drives that motor, or the wiring that feeds the motor, and not the motor itself.
If it IS the motor you can probably find suitable replacements for cheap (I'd recommend doing all four at the same time
unless you're sure they're the exactly the same motors). If it's the board then it will take a bit more work to fix, but it's still
possible to just replace the driver sections (built in ESC) if you're able to do minor electronics work (soldering and desoldering).
I saw one of these quads on fleabay for about 100 GBP that you just might want to go ahead and get.
Having TWO of everything is a great backup plan for parts whenever something breaks, fries, or just plain quits.