£900 M-ATX PC Build (Advice Needed)

i5 4690 isn't really worth it if you looking to build a gaming pc:
i5 4460 is enough
http://uk.pcpartpicker.com/part/intel-cpu-bx80646i54460


and get a better quality psu. Corsair CX series aren't that good.
http://uk.pcpartpicker.com/part/xfx-power-supply-p1650xxxb9


You could get a cheaper wifi adapter:
http://uk.pcpartpicker.com/part/tp-link-wireless-network-card-tlwn881nd

ATX vs mATX is basically a non-argument. For the average person mATX has plenty of expansion options and the boards are usually of comparable quality.

Your build is pretty solid but I'd personally shuffle the budget around a little.
I'd swap the 4690 for any other i5 (they are all cheaper). A 4460 is likely to be your best bet, but 9/10 times the best i5 is the cheapest i5.
The cheapest Wifi enabled system you can buy might actually be ITX rather than mATX as you can get a motherboard with integrated Wifi for as little as £75. This might not be suitable for you, but it's worth consideration.
The stock cooler is sufficient for a non-overclocked i5, so the 212 Evo is £24 wasted even though it's a nice cooler. You can certainly afford to save here and leave yourself the option of adding a cooler later if you want to effect the noise/temperatures.
The Corsair CX range aren't great and at £49 you can do better. The "default" cheap quality PSU is the XFX Core 550 which is cheaper and better overall. The trade off is it's not modular, but cheap+modular+quality don't really go together.

Most cases need a couple of fans, an intake and an exhaust. You can add more of course but they do suffer from (effectively) diminishing returns.
If you were using an ITX board, then you'd swap to a cheaper ITX case like the Thermaltake Core V1 or Elite 130. Neither of those really needs more than two fans to keep them cool, so the fan header is far less of a concern and a splitter is a relatively easy solution.

That said, smaller builds aren't for everyone and the N200 is very reasonably priced and should be pretty straightforward to build in. The price gap isn't going to be large either.

The Intel stock cooler is actually relatively quiet. My personal gripe with it is that the sound is ....different...to that of case fans. So if you run a case with a few 120mm fans (as many do) then it is a distinct sound that stands out from those, rather than being necessarily louder. This is pretty subjective though, hearing is in general. Personally I'd drop the cooler, give it a go and then see if you need one later. That way you can specifically target the product you want for noise/heat etc after seeing how you get on.

For some reason I didn't mention the DDR3-1333 Memory. There isn't really a good reason not to get DDR3-1600 as the prices are around the same. Any CAS9, 1.5V kit should have no issues with compatibility.

The EVGA 500B is kinda in a similar place to the Corsair CXs. Both are solid entry level PSUs and both are reasonably priced, but at your budget level I would always want to spend a little more. The XFX is likely to be the best quality you can get for the money (short of a good bargain in a sale). You could make a serious argument for spending even more than that when quality (semi)modular PSUs start in the £60-70 bracket, but it's by no means essential.

As I mentioned earlier, 2 fans can be enough so you might want to hold off buying extras and see how the stock fans get along with keeping things cool.

Just to be clear, a lot of this is nitpicking. Your original (and subsequent) builds are basically fine and there's no major problems, you are very much on the right track.

The maximum speed officially supported by H97 chipsets is DDR3-1600. Any higher speed memory will just run at that speed.

Heatspreaders can cause compatibility issues but with a single, fairly narrow heatsink like the 212 it's just a case of raising the fan a notch or two. Here's a picture of the 212 on an ITX board and you can see it's got good clearance by being raised up slightly.
This isn't a 212 (it's a 612) but it shows what I'm talking about more clearly.

There's loads of options for memory and to the main part it doesn't matter which brand/manufacturer you go for.
These are probably my pick of the moment. No big heatsinks, nice specs and reasonably priced.

Nah not really. Obviously the more you raise something up the less the fan is blowing through the fins, and so eventually it will make a difference. At realistic levels, it's likely that only the fan shroud is going to go above the top of the heatsink - the fan itself is probably still covering the same amount of area (albeit slightly higher up) as it was before.