someone not changed it to kms only idea as 50 miles is about 80 KMS you can get the odo to display in kms it is setting in the handbook.
also on perth journey 2000 miles is 3200kms
disconnect the maf and see if it starts P0113 refers to the MAF as sensor in it. The next thing would be leak off pipes and diesel check no leaks. Take cover off the engine look for wet leak off pipes or signs of diesel
have you checked out saab dealers for header tank do you have part number should be this https://www.saab-spares.co.uk/product-page/9202100-expansion-tank
welcome these newer ones need a code reader these days had the frontera about same time so much similarities between omega and frontera. like yourself learning these new motors as youngest has a meriva B
I have had quite a few vauxhalls over the years the latest a meriva B.
previously owned Viva HB, Opel Ascona 1.6 , Opel Manta GTE , Opel Mant 2.0 Sr, Frontera 2.4 LWB Frontera 2.0 swb frontera B RS frontera B limited, frontera B Olympus, Vectra 2.0 di astra 2.0 diesel and 1.7.
So brought up with all the challenges