For the last few months I know I've had a dodgy (Bestec) PSU on my HP, the light at the back of the PSU flashing, and unable to start the PC until this flashing stopped, which it would after a few mins/hours!

I eventually got a replacement (off eBay) and finally installed it yesterday. The green light is no longer flashing, the PC starts, in that the fans whir away, but then within a couple of seconds, they stop, and green light on the PSU goes off. I tried it several times, re-checking the connections, etc but each time I try its the same.

Any ideas what to check, I re-connect all the power leads as they were originally connected, and the PSU is the exact same make/model as the original...

Any ideas?

The PSU is one of the most important components of you computer, don't go cheap with this component. I would try either a Antec or Thermaltake.

Just to make sure that the problem is the PSU try this, turn off the PSU by the switch on it or unplug it. take the 20/24 pin connector off of the motherboard and take a piece of wire and short the green wire to any black wire, turn the switch on or plug the cord back in. If your fans run the porblem may not be bad. If you have a Volt meter check the different rail voltages.

You can find the color code for the different voltages here.