I am thinking of upgrading my standard turbo to a Mellet PD150 version and getting a remap. I asked my local place Oxford Performance Autos if they could do a remap specific to this engine and turbo, as they claim to be remap specialists offering custom as opposed to off-the-shelf maps, but they say they can't do it because the turbo is non-standard and doesn't match the standard ECU. Does this sound right? I don't know much about this but I though that was the whole idea of a custom remap.

If they say they can’t then find someone that can. It is perfectly possible. 

Go elsewhere, if they can't map a simple PD150 turbo then they're definitely into the flash and dash mapping