Some advice, if I may press you for your views.
I'm buying 2nd hand so I don't get burned if it turns out I'm not a strong enough rider.

So, IYHO, if I'm presented with one bike that's possibly slightly too big at a 19inch or one that's possibly too wee at 17.5 inch – my OnOne is an 18 (Murphy's Law) – yet I seem to like a little more room in the geomerty dept of a larger frame over a smaller frame; what, given the different nature of riding a SS, would you buy?

not trying to be funny but I'd buy a bike that fits. There's no point compromising if you're not certain it's for you – you'll never know fi your problems relate to bike fit or just 'cos singlespeed isn't for you.

if you there is enough latitude to adjust the fit on the smaller frame via stem and seat layback – I'd go with the smaller frame.

I can't very well buy a discontinued bike off eBay, or such like, if the seller's in Leeds 200 miles away.
I have to try, as best as I can, to make an informed decision and liking as I do larger frame size in preference to a medium-large (with my HT and my FS bikes)
However, not being over familiar with the changed riding style of a SS, I wanted to hear if SS riders on the whole prefer a slightly smaller frame over a slightly bigger frame (were it to come down to this decision) on their SS bikes in comparison to their other bikes – if in fact there's a difference at all.

go for the size bike you're used to for your style of riding, a SS is just the same as any other bike despite what anybody might say, it merely has one gear

Make sure its strong enough to carry your beard though!

I'm 5ft 9 / 10 (depending on the time of day, with a 32in. inside leg and ride an 18in. On-One In-Bred and it fits fine.

Some days I wish it was smaller but in general it fits fine.

BTW there's no difference in riding style with or without gears – you just learn to conserve the downward momentum for the upwards slog.

As usual it's the old adage – try before you buy.

Exactly what JoB said.

Why not just buy another on-one from the classifieds? There are plenty about.

FWIW, I love my inbred (853 but can't tell the difference from the standard I have to say).

I'm after a 29er SS.
That seems to be the best way to approach SS riding from what I've seen and read.
I'm 5:10:5 with a 31 inside leg.

Personally I would just convert your HT to single speed. cheap and easy to do with taking some stuff off your bike and lobbing a surly cog and spacers on. If your current bike doesn;t have sliddy drop outs then you can use a tension devise. Both this and the cog you should be able to pick off here cheap enough 2nd hand.
The best way to find if its for you is to try it on a bike that you know and that the geometry works for you.

I've SS my way through the last few winters and just normally strip the gubbins and bang a surly cog on. I've been riding my stiffee without a tension device and managed to keep tension by using a half link chain to get a better fit

Also go out for a decent ride on your hardtail, pick a gear and don't change it… sounds daft but basically the same thing and this will help you to figure out what ratio you can work with without getting loads of cogs… 2:1 is a good starting point

Cheers.
I had looked at keeping the shed a little less cluttered and swapping the HT out to a SS over the winter as it would be fairly simple. But then i thought that another steed might make swapping from bike to bike a little easier on time spent / wasted with the mechanical side of things.

It'd force me into selling 2 bikes that I don't ride, thus making space.