Whether or not a company is an advertiser on this site, it does not absolve them from the requirements of good service. As notice says, there can be good reasons for witholding a deposit. I assume that's not the case here. There also can be genuine mistakes made; let's hope this is one of them.
NAC have a generally good reputation here, notwithstanding their troubles earlier this year, and I'm sure they'd like it to stay that way. I, and many others, would be interested to hear how your complaint is handled now it's public.
Scroggs