I understand it's a serious question, and that's fine, and I am going to give you my honest answer, which I know you are not going to like!
I am going to say it could be either, but generally, I might tend to a C2.
My reasoning is:
It is domestic and could be subject to DIY at any time.
DIY'ers don't necessarily know about, nor understand safe zones.
There is not necessarily adequate mechanical protection over the cable to prevent drilling damage in the event of a DIY incident.
The cable might or might not be in safe zones.
The spark is legally bound to comply with EAWR even in a domestic premises.
The consequences of a DIY accident could be devastating to a family.
I would err on the side of caution.
This minimises my risk, why should I take a risk on something when I feel I could justify this.
The installation is coming up for 40 years old so approaching the end of life.
Would I argue against someone who put a C3, I would have to say it depends.
Sorry, if I were the expert witness to the court following a DIY fatality and the cable was in zones then I might not be able to justify a C2, IF, the inspector could put a rational argument forward. The defence from the inspector would be I suspect that the cable was in zones, but, the counter-argument I would put would be, how can you expect a DIY'er to know about zones.
If the cable was out of zones, I could turn around and say, you were doing an EICR, you couldn't confirm the wiring routes, so why did you not code C2 in the knowledge that this scenario was possible, and could result in a fatality with no RCD fitted?
The issue we have is the absolute duty under EAWR reg 16, it would then be down to the inspector to construct a Reg 29 defence.