It could also be the RAM as well, but you got no beeps though. Check for any stuck buttons, loose connections, etc.

*JM*

I've already checked, making sure all the wires of my PSU are firmly connected and the various components are properly fit into place.

The best thing you can do in this case is to remove the mainboard,CPU and RAM and PSU and set it up on a wooded desk(non metalic) plug it all in and cross the 2 power switch pins with a screw driver of a second and see if it boot or if the same this is the last thing you can try and a lot of the time it can help sort out whats going on because it removes a lot off posible issues e.g shorts and power switch.

I can try this but I'm not sure which bits I cross exactly to make it power on outside of the case. Can you provide a video or an image to give me a better idea on how to go about doing this?

Hey, I'm guessing you ment Mobo (Motherboard here)?
Replacing the PSU and CPU ( Do you mean motheboard maybe) did not change the result in any noticeable way unfortunately.

If that's the case your next port of call would be a new CPU. Make sure it's compatible with your MoBo, if you give us it's make and modle number, we'll be more than happy to help you with that.

My apologies, I did indeed mean the mobo and not the CPU.

This is my question basically. My friend said the only way to know is to replace your PSU, Mobo and CPU in that order from most likely culprit to least likely (or most expensive to replace). As stated, I have replaced the PSU and Mobo yet nothing has changed in my situation. A new CPU is going to cost me quite a bit and I wanted to know from you fellows if the CPU is very likely the root of the problem here, thus making it worth the investment of a replacement part.

I also have a secondary question here relating to the replacement of the CPU - is it wise to buy a CPU that is compatible with the shoddy replacement Mobo I purchased or should I assume my old Mobo is fine and buy a CPU that's compatible with that?

When I bought a replacement PSU, the new one was better than my old one so I actually kept that. My replacement Mobo on the other hand was a £30 Foxcon part which I purchased due to it's compatibility and price, just to see if a new Mobo would get the PC working. So what do you guys think, a new CPU that's compatible with my new or old Mobo?

ALSO...

I have a YouTube video here which I recorded last week for my friend so he can see that I've definitely plugged the PSU sockets into all the necessary places. If it helps in any way, please do give it a quick watch. Excuse my voice, I had the flu at the time so I was half dead. :]

the pins you cross for a second are the 2 where the power switch for the case connects to the mainboard. The reason I ask you to do this is it's very strange you get no beep codes at all with the new mainboard and PSU you should get some even with dead CPU