Accor have finally decided to respond to me again, this time with a new excuse, perhaps even worse than any previous excuses:

"The amoma.com sale conditions are flexible while as you benefit from the better rate ,your AccorHotels booking is non-flexible one (non-refundable and non-cancellation sale conditions).

I inform you that the sale conditions regarding the other website have to be equivalent to those suggested in your AccorHotels booking to meet the eligibility conditions of the Best Price Guaranty."

So what they are now saying is that if another website's rate is cheaper and MORE FLEXIBLE, they will not honor the BRG - this has to be some kinda bad joke!

Meanwhile the advertising standards authority here has started to look into this.