Digital photography

Are you doing any post processing? Your shots have a certain hazy, flat feel to them which I really like.

Cheers. Yeah, I use a couple of presets in Lightroom but have tried to dial it back a bit. Mostly just turning the highlights down and tweaking the colours.
The out-of-camera jpegs can look like this too with the right settings.

The vsco filters will do a good job of it as well

Yep, I use them a lot.

It's a fence!

The angle of the wall in the background is confusing me. Is it a tunnel or just a wall at an angle?

Well, is it?

No, it's not.

It looks like an out-of-use pathway.

Hobo - you can get pretty close with in-camera jpegs, you just need to fiddle with picture styles (Canon, that is. Not sure what they're called on Nikon).
Here's one straight from camera with no editing whatsoever apart from a slight crop. It's just very flat, low contrast, low-ish saturation and highlight tone priority enabled. Plus I tweaked the blues to be a bit less blue...

a couple of shots I took from my balcony during the stormy days

Amazing. They look like CGI.

I love the Leadenhall building

I bought a lens from someone on here a few months ago but until now I've not really had much of an opportunity to play around with it. However now I'm looking at the pictures it seems that at shallow DoF the pictures are consistently slightly out of focus.

For example here:

Full size here, f1.8, 1/200, ISO 100, Flash.

In this case I used auto focus and a single focusing point directed at the subject's left eye. As you can see however, the eyes are not sharp while the plane of focus is a couple of cms further away from the camera. I haven't yet had an opportunity take photos of a ruler with the camera on a tripod but the same effect seems present whenever I shoot with a shallow DoF, and I've never noticed it with any of my other lenses.

What are the likely sources of this error and what can I do to try and diagnose the problem?

My first suspicion is whether I've simply bought an incompatible lens, the camera body is DX while I believe the lens is FX compatible (Camera: Nikon D7000, lens: Nikon AF NIKKOR 50mm 1.8D). Might this be the problem?

Alternatively, is the fault more likely to lie in the auto-focus mechanism of the camera body or somewhere in the lens and how do I find out?

I should add that I've not yet discussed this with the seller (although they may read this) and I'm not accusing them of selling me a dud (at least not intentionally), but rather I'm just trying to identify whether a problem exists and whether or not it's caused by my own ineptitude.

DSLRs often focus slightly out at large apertures. You can fine-tune the focus for individual lenses with newer bodies, including yours.

http://farbspiel-photo.com/about/gear/nikon-d7000-quick-tips#d7000_tip001

Shooting at f/1.8 can be a bit hit and miss but it sounds like you just need to do a bit of af micro adjustment. Nothing to worry about, basically.

well, is it?

Well, maybe.

Maybe it is, maybe it isn't, well is it!

Sorry but your screenname is just.. I don't know

Different.

what can I do to try and diagnose the problem?

..shoot a ruler or something, different things, to make really sure it's the lens.
Fine-tune the lens in-camera as h2o said, and repeat.

Another thing to try would be focusing on something at infinity and minimum distance and see if Theres softness at either extreme. That will give you a starting point.
Also, at f/1.8, I'd expect softness anyway. What's it like at say, f/2.8?

As above, expect some softness wide open.

You say you used AF with a single point on the subjects left eye. This means you either used something other than the central focus point - which in my experience can be less accurate - or you locked focus and recomposed - which can definitely cause problems of it's own. Even if you breathed between locking focus and shooting you may have moved enough to put the focus way off when using a large aperture like that.

My boss wanted me to give opinions about getting a new camera kit, ideally a FF body, he would like 2 lenses (a standard zoom and a wide angle), user friendly as it'd be a team camera and some colleagues don't know much about photography, HD video, plus a flash and other, in my opinion, unnecessary accessories. His budget is £2k. After seeing his wish list, I thought erm... £2k? FF? Hello...

A fellow colleague said, FF and 2 lenses, £2K no problem...

Think I have managed to talk him out of getting a FF, so realistically, what would be a sensible option(s)? I have in mind either the Nikon D7100 or Canon 70D, plus a 24-120 F4 or whatever the Nikon / Canon version is, and a as wide as possible / affordable prime.

Problem is we can only buy from places like Calumet... so £2k isn't really a lot of money...