I am looking to purchase a new camera with the idea of using it for portait and wedding..

Scott, it's kind of hard to answer your question without having much more info. So... What do you have now, and how does it hold you back from doing this? Is there a reason to only look at Nikon? If you already own a Nikon, which lenses do you have? And flash etc.?
What's your budget?

Hi Scott & welcome! As Wouter says, we need to know a little more. Having said that, a lot will depend on your intended lighting solutions. Natural light would suggest good high ISO capability and fast lenses. For portraiture, prime lenses would make most sense. Remember that for weddings you need to budget for two bodies, two flashes, lenses to cover the relevant focal lengths twice over etc. The D7000 has dual SD slots - a nice insurance policy for weddings. The D700 has the better low light capability.

Figure out what lenses flash(s) and other accessories you need, then get the most body you can which what you have left in your budget. Two memory cards slots are advisable for event photography - the D7000 is Nikon's most affordable body with this option.

The D800.

A number of currently available Nikons can be used very nicely for portrait and wedding photography. D700, D7000, D300s, D3s, D3X etc. Now, some of these are being replaced by the new D4 and D800. Depending on your style of shooting, and how much in a hurry you are to get the cameras, you can choose any of those cameras. A D800 or D4 might be best, if you can afford them, but they are not availble yet for some time. A D7000 would certainly suffice, as would the D700. If you want two formats, get a D7000 now, and a D800 later when it becomes available. Also consider a second hand D3s ... that would be a killer for low light shots if that's your thing. If you mostly shoot with lights (i.e. flashes) and in daylight then a D7000 would probably be the best bang for buck, but the FX cameras will give something of an edge in low light if you want to include that in the pictures.

Firstly, thanks for your responses.. I currently am using the Nikon d3100 which I use for landscape. Ian considering upgrading to the d800 for landscape.

I have sigma 10-20 wide angle lens, standard 18-55 which are the main lenses I use. I very much like Nikon and the ease and have little experience with others.

Budget is ideally under £2000.

Scott,
Both your lenses are DX... getting a D700 or D800 means also you should be looking at getting new lenses. Not going to happen under 2000 pounds, I fear, as the camera alone costs that amount. For the D7000, it would be less of an issue, though your current lenses certainly are not ideal for weddings.
And it still doesn't answer one of the key questions: how is the D3100 holding you back? What is it that you need which it can *not* do? If you upgrade, it should solve a problem.
There is not a single reason why the D3100 cannot take great portraits. It can be used for wedding photography too - it can deliver the goods. So, in upgrading, what are the "must have" features of this new camera for you?

There is absolutely nothing wrong with wanting a new camera. Seeing the D800 specs also make me wanting one... but fact is: the more I think about it, the more I know the D800 does nothing to advance my photography; the body I have is still more competent than me.
The problem with the D800, and also the D7000, is that it will need good lenses to make the best of it, and to a lesser extend the D700 also merits good lenses. It will drive up costs tremendously. And your photos will not look better by definition because of this gear. If you're not pushing the D3100 to its limits, a D7000, D700 or D800 is not going to improve your photos much.
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More specifically;
For portraits, the key is light and lighting. Rather than a new body, consider flash, studio lights. The 18-55 can work just fine (at f/8), though you may want to consider getting a lens better at portraits.
For wedding photography... to be honest, your budget won't do it, I fear. You need 2 cameras, 2 flashes and faster lenses (f/2.8 zooms are common), plus the usual bucketload of memory cards, batteries - if you want to have a go at it professionally, or at least well prepared enough to deliver professional looking results.

Hi Wouter,

Thanks for you feedback and it makes sense.. With regards to my d3100 the images are sometimes not sharp enough and slightly pixelated. However when I do landscape I don't have this issue at all.. When I shoot landscape I use my lee filters and none whilst doing weddings, any advice on what lenses would be better for weddings?

I agree with the lighting for portraits and this is something I have played with and consistently got better.

It's been kind of bugging I have had very good results with landscape in a short time, and the wedding stuff is taking longer than expected.

Really appreciate your help.

The sharpness and pixelisation... it could be a lot of reasons... Landscape work is typically in plenty light, and taking time to frame, get proper exposure, stabilise yourself (or better, the camera on a tripod). Weddings are frantic, frequently indoors under difficult light conditions, and you have little time to get the shot. In short: in a wedding, there is so much that can go wrong.
Underexposure and flat light both can cause images to look noisy and not very sharp. You need good contrast to have good apparent sharpness - this really comes from having good light. I'm no expert at weddings, but the bit of work I did: I used flash where and when I could. It made a lot of difference.