Una nudged the man who wasn’t supposed to be there. With what? How? After finishing the whole thing, this is neither interesting as a first line nor important imo; see whole reply for my suggested start.
“Right. Listen. Here’s what I suspect happened…” not a big fan of the ellipses to end dialogue] she held an index finger aloft, conspiratorial, [she held up a conspiratorial index finger? hard for me to imagine a finger as conspiratorial. I imagine you mean her tone was such. and with her other hand pulled him closer, where she wanted him. I'm assuming it's where she wanted him, since she's moving him.] She paused. She paused [to notice the gooseflesh...]
Noticing the gooseflesh on his bare chest, Una yanked the blankets tighter about them both. It was warm as toast under the coverlet on her old iron bedstead. The rest of the bedchamber
, of course, would be arctic by now; the water in the wash-stand jug frozen solid and the morning sunlight merely soaking through the curtains like tepid soup. over-written, possibly just the phrasing of it] The Scottish weather seemingly had not troubled itself to notice it was supposed to be May. oh I love this! really establishing a voice I can appreciate]
“I think they only fired me on a flimsy pretext, yesterday. I have suspicions. The firm was in debt, I’ll wager, and could not afford everyone’s salary. So they dismissed me, since I was the most junior clerk,” Una folded her arms across her naked front with an emphatic nod, pleased at her theory. “Yes, it was plainly all a humbug.” [ 'fired me' ' the firm' 'salary' seems phrased in such a modern way compared to the rest of the language and setting]
Nodding everyone is nodding then], the lad tucked a dark rat-tail of hair behind his ear. His tresses had looked more agreeable at Drummond’s gin-palace last night, pinned up in a knot with sprigs of cheap herbs and paste-jewellery ornaments. great image of this chump; I really like this as a reader] This morning, he didn’t look old or anything nasty like that - Una judged him twenty-something-or-other, same as herself - but the great Lady above certainly still made males that were better by candlelight. I daresay I shall recall his name in a minute… This whole last part could be more clearly phrased imo. It's jarring to read.]
To make a suggestion, I have recently been focusing on the weaving of character, setting, etc. I feel as though if this is the first 250 words, you might explore these suggestions based on some templates I've made from reading Sanderson, Rowling, and Patterson. Helpful or not, for the first 250, I feel like I wish I knew a bit more already.
Character doing something to start scene; typically a change in position. This would help you reveal earlier that they are in a bed. I was imagining them standing until a few sentences in. I didn't like having to re-imagine the whole scene up to that point. One rule I tell my students is I should know how they are moving (not moving) before I have to watch them talking.
Dialogue that shows personality
I loved your 'pleased with herself' but I didn't feel any personality came through besides that. When she said 'what I suspect happened, I really was imagining something modern and possibly more serious than getting fired.
Character jokes or acts in cute (whatever their emotion or personality dictates, but it should make the reader want to like them)(or not like them if that is how I'm supposed to feel about her) way
Besides noticing the goose bumps, she doesn't DO much besides nod. Is this a one night stand? Is she attract to this dude? Does she think he's stupid and is just using him as a sounding board? I don't understand how she feels about this dude, but with some action I'd be sold.
Use her perspective to describe 2nd character’s looks and have them watch the other.
You do this in yours with his hair, but you could throw in some eyes? I wanna know how she sees him, since you go on to say the 'better by candlelight' part (which I love)
They start to move around the space
Hope this is helpful at least for a second go at it. In all, there are some nuggets of gold, but much to explore about this important moment.