If it does turn out to be the regulator on the HPFP it is not an hard job to sort out, and the cost of the part is minimal.
It takes about 45 minutes to change as the starter motor as to be removed to get the regulator. Couple of photo's of the regulator.
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2
https://www.ebay.co.uk/itm/332911972...m=332911972741
Leak back test kit, try this first or see if you can get onto a T4
https://www.ebay.co.uk/itm/6-Cylinde...t/264029762969