May I take it that the 5 in the NGK label NGK R ZFR5F was the heat rating? I note Haynes prescribes Champion RC 10MCC R6 plugs for the Astra 1.4 and 1.6 normally aspirated, NGK IFR7X-7G for the 1.4 turbo, and NGK ZFR6BP-G for the 1.6 turbo, very specific.
The plugs I removed were well eroded, and I imagine were original. As said before, modern cars are wonderful. The Senator was not so good, electronic ignition, but with a distributor at the front behind the open cheese grater grill, which allowed the rain to blow in.
That's right the '
5' on the NGK is the heat rating.
As I see it the Vauxhall EPC lists two possible factory fit plugs for your 1.6. Bosch FQR
8LE2 and Champion RC
10MCC (aka OE198/T10). These and your NGK ZFR
5F are all equivalents and are single pole Copper/Nickel. However they all use different heat scales and counter intuitively a lower number is hotter on the NGK scale. Hence Bosch 8 <=> Champion 10 <=> NGK 5. It's never a precise equivalence.
The turbo plugs are colder and are Iridium and Platinum.
So likely somebody changed the plugs in your Astra for the NGKs some time ago. Seems probable they were not factory fit but aftermarket.
I wish I still had my Senator.