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Here's what I bought last February, it's been torture tested, and I recommend it.
Here's my review/breakdown.
I'm not that big (not that small either) but I have big people who visit. This has a 350lb weight limit, and the kicker is that instead of securing the back to the seat with just a few bolts, there is an L-shaped steel plate that secures everything together.
The base is hefty, it and the arms are made of what seems to be fairly solid wood. It's held up to my large visitors, my wife sitting on my leg to look at all the dumb shit I find on the net and reddit that I make her watch, and both my children clambering all over me at the same time when we play games or look stuff up together.
I play games on PC and Xbox by leaning back, kicking my feet up on the desk or TV stand. I get up and down a lot. It's good for hours of vegetating, although you will get sweaty if your room gets hot.
The bonded leather seems to be holding up well, no rips, no tears, no real wear on the seat. The cushion is memory foam, and the arm cushions are the only thing with any remarkable divot. Hours and hours of elbows pressed into them while holding a controller.
It's a Serta 43506 Bonded Leather Big & Tall Executive Chair in the color brown. I love it. I am not necessarily tall (5'10") but this thing is so comfortable. Not a single complaint.
Here is the link: http://smile.amazon.com/gp/product/B00AVUQQGQ?psc=1&redirect=true&ref_=oh_aui_search_detailpage
If you're having substantial back problems I would advise getting an x-ray to see if you have scoliosis and may need surgery or to visit a chiropractor instead of a therapist.
Most thick executive chairs are good for sitting for long periods but you also need to get up and stretch instead of long gaming sessions.
This is the one I use. I am in the US so you may need to see if its available in the UK or EU where you're at.
Here's Amazon's best sellers: http://www.amazon.com/gp/bestsellers/office-products/1069132/ref=sr_bs_1
My personal chair is very similar to this. http://www.amazon.com/Serta-43506-Bonded-Leather-Executive/dp/B00AVUQQGQ/ref=zg_bs_1069132_8
Also, I'm 6'0 and 220 and it's big for me. I definitely think a good chair is a big part of PC gaming. Good luck finding one!