Thanks for the mention guys, and for the message
@Richie56. This is going to be difficult to really point in the general direction without actual fault code(s) extracted or read by the garages at the point they had the vehicle in and cleared the system. The warning on the dash is merely a warning for end user and doesn't give any detail.
The issue I've come across in my own experience was on my Euro 6 Insignia, when I first owned it had an intermittent Adblue issue which turned out to be poor/resistive connections to one or both of the NOx (Nitrogen Oxides) sensors bolted into the exhaust in front and behind the AdBlue catalyst. This fault was covered in a Vauxhall bulletin, but for Insignia only from what I can see, not Zafira-C.
There is another bulletin which IS listed for your Zafira - in which the testing of the AdBlue system confirms whether the correct amount of reductant is being provided at the injector into the exhaust. All this is going to require a fault code based diagnosis however.
I will say at this point that not all AdBlue is the same - the fluid should be labelled of ISO 22241-1 standard bare minimum (don't get putting anything in that's not labelled as such). However, even the ISO compliant stuff can be of mixed concentrations. Sometimes either weaker than the specified concentration of 32.5% urea, which can give symptoms of much higher AdBlue consumption. Worse still is reductant that is TOO concentrated - this stuff crystallises over time anyway, incorrect concentration can cause the lines and specifically injector to clog and faults like this to occur.
A quick, very rough test - if it stinks of cat piddle, it's probably slightly too strong. I used to use TPS's Quantum reductant, near odourless.