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The Nikon d3100 does not have an external microphone port, which is normally not a problem (built in mic is great) unless your doing AF-A, then you get lens noises. For a shot that requires zooming and focusing, you get some noises from the lens, which I want to avoid.
What would be a good external audio recorder that I could hook a shotgun mic into? So when I am doing cretin kind of shot I could record on the external mic and then sync the audio up later
The H4n has a decent heritage as a consumer-level device, but you may wish to consider slightly more expensive gear. Depending on your budget, you can go for a Nagra (Swiss-made, a venerable heritage as THE highest quality reel-to-reel manufacturers and today they also make very pricey but excellent - solid state units). If you don't have a blank cheque to hand over, you could also consider Edirol (Roland), Fostex or something very handy like a Marantz PMD660 or Tascam DR-100.
I myself own a Zoom H2 and whilst it's excellent for the price, it does have a few shortcomings:
If you're in the UK, somewhere like Solid State Sound is a good place to start when shopping around, they have tech specs and decent images for loads of devices.
As a final consideration, if your audio inputs aren't quite suited to driving a decent mic but you still want to print the audio to tape along with the video (as a backup if anything else) you could consider something like a juicedLink unit to preamp the incoming sound. They also make a unit (the DS214) specifically for use in conjunction with devices like the H4n to give you a dual system setup (read: both units simultaneously record) which looks particularly interesting.
(Update, July 22nd 2011)
As an adjunct to my original answer, if you want to listen to a quick 'n dirty "straight out the box" recording comparison of all the reasonably priced solid state field recorders available today... DV247 just did one! It's hardly scientific but should give you a half decent idea as to how each unit 'sounds', plus a few pros and cons. Take a listen at http://magazine.dv247.com/2011/06/17/portable-digital-audio-recorder-comparison/.
While recorders with built-in microphones like those from Zoom are quite popular, I consider them badly-suited especially for video purposes. You definitely need the possibility to plug in decent external microphones, and then it's questionable if you will ever need any internal microphones at all.
If you can afford it and don't mind carrying the weight, get one of those professional field recorders like Tascam DR-680, Fostex FR2 or even a Nagra. They give you uncompromising high quality, reliable performance and good handling, for instance through well-operable gain controls. You also have multiple channels, which can be useful in a variety of situations.
If you appreciate low weight/size and affordable price (if only to be able to spend more on microphones, which can arguably contribute more to a good sound than any preamp/recorder), and only need 2 channels, I would recommend the M-Audio Microtrack II. It is really lightweight (and pretty cheap) and might still give you everything you need: professional mic preamps with phantom power (though only through ¼″ TRS rather than XLR, but fully-symmetric), and decent ADCs with 24 bit 96 kHz. You probably want 24 bit, because even with good analog gain pots you should always leave a bit headroom for field recordings unless you don't care if the limiter sometimes cuts away the dynamic range, and with a small recorder like the microtrack even more so as it's quite difficult to set the gain precisely. With that specific model you also have the drawback of a not consumer-exchangeable lithium battery, so you may need a portable USB power supply.
