Gaz, I looked into this in some depth it ended up that most of the UK companies were charging around £600-800 to modify a k03S to use K04-023 internals ( that is assuming that your old turbo is in good condition, Turbo Dynamics said they have a skip full of returned customer units as 80% of them had cracks in the turbine housings, straight purchase being £1200) which is good for around 260-270 bhp. RobDon on here had a Ko4 Hybrid and pushed it as far as
290+ bhp but it didn't last long, Star told me that the turbo was good for a reliable 260 and no more, they used Turbo Dynamics MD 349 turbo's.
To get the benefit of this you need to uprate your MAF, injectors, FPR and Fuel pump to TT/s3 Ones and custom remap to suit.
JBS do the JBS04 which with the right mods will see knocking on 300bhp but it is expensive and is built to James's specs which are a closely guarded secret.
I am going with a LCR K04 as it will work out cheaper than the hybrids but give slightly more, 270-
280 should be realistic.