When I'm hacking at home, I use a really cheap Ikea chair. My chair at work isn't much better. I don't think I can persuade the boss to get me an Aeron, unless I get a doctor's note saying I have a back problem (I don't).
So, what are some good recommendations for programming chairs? Break it down into price ranges (e.g. under £50, £50-£150, etc.).
I use a simple hard chair that doesn't swivel, doesn't turn, and has a very thin cushion. It supports my lower back and is great for leaning back in, or sitting sideways. The chair has broken countless times and welded together. I can't stand sitting in anything that is supposed to be more "comfortable." I find that those chairs are not good for long term sitting.
I've got one of those generic black "leather" (not real leather) manager's chairs at home. It was £60 from Ikea, but I think they have since stopped making them. Its somewhat worn out around the edges now.
I've noticed that Staples are doing some nice chairs at the moment, you'd be looking at between £80 an £150 for some of the nice leather or fabric covered ones with arms and adjustments etc.
I got one of the Staples chairs in brown leather, about £150 as I recall. Looked good in the shop but not very comfortable for long periods (thin cushion) and has non-adjustable arms which jam under my curved desk if you are not careful! Not one of my better buys..
I don't work much from home these days but if I did I would seriously look at the Aeron - only twice the price of mine and (reportedly) much better for your back. After all we only get one spine to last a lifetime, might as well take care of it.
They say fitness balls are great for your health. And Google uses them at their offices.
We've got Humanscale chairs at the office. There okay for the back, but I'm sort of short (5'7") and they don't give me enough thigh support, nor is the amount of lower back support adjustable.
I have the Alera Eon Mesh Executive Chair which I bought from Staples. It sounds like you have Staples in the UK as well so you might have good luck finding this chair. I wanted this chair because I liked the idea of a mesh chair but couldn't bring myself to spend the money needed for a Aeron chair.
This chair is reasonably good and comfortable. I especially like the lumbar adjustment which even my chair at work doesn't have. I got it for under $300 at Staples. I would recommend trying before buying or get a good money back guarantee since most chairs feel good for the first hour. It is after sitting 3-4 hours straight that you notice a difference.
After using a staples chair for years I just went out and bought an Aeron. In my opinion they are worth it. I offered to buy my own furniture at my latest job. I feel that strongly about my health, comfort and productivity.
I was on a budget for home use since I don't spend as much time on the computer but needed a decent chair, and I read somewhere that IKEA had a nice simple ergonomic chair called Joakim:
http://www.ikea.com/us/en/catalog/products/90096456
http://www.consumersearch.com/office-chairs/ikea-joakim-swivel-chair
Cost is around £100 / US $170.
It's probably the simplest ergonomic chair you can get but it keeps me in a much comfortable position than getting some executive leathery office chair or a cheap computer chair from Office Depot or Staples, and that's all I wanted.