This is truly an enticing vision, involuntarily raising the pulse (and something else as well). Particularly nice job on the warm tones of the background, and the realistic flesh tones and shadowing of the characters.
Caregan, I "love what you've done with the place" - with the "place" being the stereotypical richly-uphostered, well-stocked mansion library full of rare leather-bound tomes, to which the smoking-jacketed men would retire after dinner to consume expensive cigars and snifters of aged brandy while holding court on all of the various and sundry issues of the day ...
... but your modern kinky take has the dull fuddy-duddy blowhards replaced by two smoking-hot women in a situation that would have made those plain vanilla Victorian/Edwardian/Whateverian gentlemens' florid faces blush to the very limit of their rapidly receding hairlines.
This pic fairly screams for what Hollywood screenwriters would call a backstory, but you wisely elect not to provide one, leaving it up to the viewers' most sordid imaginations. The brief description ("Never accept a dinner invitation from the mistress unless you are very sure of what you are getting yourself into") seems to imply that the blonde gal's situation is not voluntary on her part, so how was she so thoroughly subdued?
She looks rather fit to have been subdued one-on-one, unless the mistress is a mixed martial arts steel cage champion. Failing that, did the mistress drug her, blackmail her, or did the mistress receive help from additional (now dismissed) servants? Inquiring minds want to know!
It is very hard to get a read on the mistress' mood by reading her expression - which probably makes the situation all the scarier for the poor blonde hottie. The mistress' face gives absolutely nothing away - it is neither cruel nor kind, engaging nor dismissive, empathetic nor contemptuous. It is the Mona Lisa without the enigmatic hint of a smile. It is simply ... impassive ... like a research biologist about to dissect a frog for the thousandth time. If she were sitting at the final table in the World Series of Poker, no one would know if she were holding a Royal Flush or an utter bust.
The smoldering sexual subplots really stir things up here. The breathtaking sight of the skintight pink undies on the captive's perfectly-formed ass (or is it 'arse', seeing as you're from the UK?) would make Pippa Middleton green with envy. The slipped-off shoulder strap on the mistress's fetching black designer cocktail minidress, not to mention the riveting position of its now-uber-raised hemline ...
... awww, screw the review. I've really gotta go take a cold shower!