Any Recommendations For A Good Chair?

This is upright when you want it to be, and very comfortable; about £150 including VAT, and they do free delivery within about 24 hours, if you order early in the day: Helsinki Executive Office Chair by Viking. But I suspect from your question that you may want something "more upright" than that? They do have other good ones, too (but some of their cheapest are also visibly cheaply made, and maybe to be avoided). They're a decent and reliable company to buy from, anyway.

This is my chair...

Broyhill Big & Tall Traditional Executive Chair with Wood Accents

Oversized and comfy.

I don't know your budget, but if you have $1200 or so to spare on an office chair, I'd recommend getting the Embody - Office Chair - Herman Miller

I have a friend who's an IT guy, and he uses it. He's very pleased. I've sit in for a little while and it's comfortable, but to see if a chair is really comfortable on the lung run, one must use it a few weeks or so, therefore don't take my words for it.

Anyway, it has a lot of adjustments, and it's very durable. The previous version, the Aeron chair, came with a 12-year guarantee (if I recall correctly), and there are people still using it after more than that. (I think I've heard someone was selling an Aeron chair after 20 years of use on eBay.)

There are also cheaper alternatives around $200, but you need to scour the web for reviews. You could probably search in IT forums because I'm almost certain they have such discussions.

Anyway, I personally wouldn't go with a chair Alexa or Dennis recommended, because even though they are comfortable for the first few minutes, most of them don't have lumbar support, or it's not adjustable, or the seat depth is too long for me and it becomes uncomfortable after a while. (Not to mention the back pain one can get because of an incorrect seating position.)

Edit:

The website Alexa linked to has a section called "Ergonomic Office Chairs". You might want to take a look there, as well. I'd say if you spend a lot of hours at the computer (as I assumed), you'd be better off with an ergonomic chair rather than a regular office chair.

With regards to buying a chair, I recommend going on Amazon and buying one that has great customer reviews and that is within your price-range.

I figured out my desk was too low. So I raised it. Lowered my chair. Prop my feet up so I can lean back. And wow. It's actually quite comfortable. Can't believe I used a standard height desk for so long.

Why don't you try a pillow for your seat and a back cushion for your posture, which can work for you if your chair is too big. You can get back cushions with elastic bands on them to wrap around the back of the chair to hold it in place.

Other than that, I suggest you personally go to office supply stores and sit in the different chairs to find something that is comfortable. Check with the clerks to find out when there are sales prices available. You will probably have to spend a little more to get something where the height and back are adjustable, but if it takes care of what you need, it will be worth it.