It's just been explained to me that Behr make the parts in this case.
The OEM will have Behr or nothing on it, and the OE Porsche Brand will have the official logo on it, for £100 extra (from Design911). They are in fact the same part.
I spoke with
Northway in Reading, and they'd be happy to fit any part not supplied by them, but then would obviously not warranty the work, as it could be the part at fault. Otherwise they'd supply and fit, either of the parts listed above, or from Porsche, whichever I choose.
The Porsche items inc labour are £1080. From OPC and labour is £1200, for both.
If I wanted to use the Behr OEM parts it'd be £400 cheaper, with the 1 years warranty on the part (not labour).