Typo corrected!
The odd thing is that after I challenged not being given an invite the agent stated I was entitled to access, but no guest (as they had been told Star Gold is the same as Voyager Gold). This despite the Star Alliance website stating it is me plus one guest when on a Star Alliance operated flight. If i was entitled to access as Star Gold, then the lounge must have been considered as Star Gold lounge.

I've never been denied lounge access on the basis of a lounge not being operated by the relevant airline. LH, OS, LX to name three have given me contract lounge access across Europe on the strength of Star Gold. That said, there was no Star Gold sign outside the WDH lounge.
I'd prefer to complain to SAA HQ rather than the local team, as my request to speak to the station manager was met with "he agreed the policy, ask at the ticket office if you don't believe me". Do emails to the customer care address get looked at, or is snail mail a better option?