Thanks for all the replies gents.
Enormous thanks to @nissannewby & @MudRunnerTD for the phone calls and guide.
Cars been limped and parked for the night. Ran out of daylight.
Suspect boost controller as there is something rattling inside. Spring and/or ball stuffed. Have replacement ready .

Also other hoses to check thoroughly and a few tricks Mat and Darren pointed out to try in the morning...