Hi Judd - your perspective on this is informative and helpful - thank you.
I can buy Bosch manufactured PDF sensor sold under the Subaru part number 22627AA500 for about £70 inc VAT as a member of the public from any number of parts distributors without any trade discounts. Subaru UK charging £368.35 for an equivalent/same part to the trade seems beyond excessive. However the situation as you describe it is correct, the garage went straight to Subaru to ensure that they get the right part that they know will work correctly on the vehicle and have some guarantee if it doesn't. It is what it is, and as you say you pays your money and takes your choice, it's just that in this case I didn't get any choice. Thanks again for your input and rationale.