Why this is called "Mistresses" is a puzzle, because it's about four women, three of whom aren't mistresses
Except, wait. Ah, I see. It's a salacious title and we all have to merchandise I suppose. The series itself? Delicious. Most of the characters are hell bent on cutting metaphorical chunks off themselves. Great fun. Reminds me of LWT's 1976 miniseries "Bouquet of Barbed Wire", where every character and their dads wielded the machete.
Siobhan (Orla Brady) is the only actual mistress and she's getting herself in proper trouble. Husband is infertile. So no chance of a baby there then. But at work, there's Dominic, played by uber-sexy Adam Rayner, who looks so good that there's no surprise when heavy lust breaks out. Dominic, it turns out, has no fertility problems. I expect his sperms do swashbuckling - probably each carries a little sword - and now Siobhan is inevitably pregnant that way instead. What Siobhan should do is to shut up good and tight, and go with - it's a miracle. What she actually does of course is say to her friends "I have to tell my husband". No you don't. Really, you don't. Stop. Stop!
Katie (Sarah Parish) was a mistress once, for we learn that she'd had an affair with a married man before the series started. Unwisely, she's now taken up with his son. The father died of cancer and Katie, who is a doctor, helped him on his journey. So, she's an euthanasiarist, has had affairs with two of her patients and is sleeping with the son of the father in carelessly ignoring the incestuous undertones. It's not going to end well for Katie.
Jessica is an experimental lesbian. She arranges events and she's busy doing a lesbian marriage as the series gets underway. She quickly gets into steamy eye exchanges with one of the brides Alex, played by Anna Torv, and the script hurries along to a lesbonk with great haste. However, it can't think of a good way of putting the two women in bed together, so it invents a very lame "You're not having a hen night?! Well I can't let you get away with that. I'll organise one and the guests shall be .. Me!", which achieves the result but isn't exactly Winterson. I thought script writers were supposed to earn their living. Lazy. Torv's interpretation of her character is good. Alex treats Jessica as possibly unsavoury and Alex's body language always points backwards when she's moving forward. Mind, once she's over the wall, in she happily goes, and unsurprisingly, for Shelley Conn is so mouth-watering that so would a good percentage of the human race given the chance.
Which brings us to Trudi, who's a widow. Of course there should be plain people in a community but, if you're going to have a plain character, then you have to invent something to admire in them. It's quite possible to be a lump and engaging. However, Small's Trudi looks like one of those characters that Casting put prominently in a medieval crowd scene after the director said "That's ridiculous, not every single character would be pretty". More tellingly, you can't find anything to admire in her. Nothing. She's a turnip in a bowl of apples. Appallingly, she does "sexy" from time to time. I won't forget her appearance in bright red corset with stockings tucked into her crotch for a long time, and for all the wrong reasons.
So, good, delinquent fun all round. There's easily enough material here for a second series and I hope that they do one. I trust they learn one lesson though. The characters never take off their underclothes in bed! Having spent abandoned hours of unbridled lust, afterwards they surface still wearing their bras, or keep the sheets tight wound round their naughty bits. After several episodes, no nipples yet and you can go beg for a cock. What's this? Early 21st century puritanism? So production team, are you listening? Your characters will solve some of their mental problems if they Do abandon as well as Talk it. At least they'll have more fun in the fun scenes, poor things. In a series dominated by either being in the bedroom, or wanted to be in the bedroom, or having just been in the bedroom, it's a bit silly, and jars a lot, that the characters bonk in their underwear.
Overall. I was going to score 6 (top end medium), but the series does one trick that's rare enough. When each episode ends, you always want more, and you look forward to the next one with anticipation. So I score 7.