MacBook 5,1 Late 2008 RAM Upgrade to 8GB

Hi everyone, I have a MacBook 5,1 running El Capitan, currently on 4GB of RAM. I checked online and saw that the 1.4 upgrade to the EFI firmware brought the ability to expand the RAM to 8GB. I ordered some Mac compatible DDR3 1066 SODIMMS (Corsair, MacMemory), 2 x 4GB and installed them. However, the machine will not boot up with the new RAM installed. It stalls at the grey screen with the Apple logo and the progress bar at approx 2/3. If I remove 1 SODIMM it will boot up on a single 4gb stick, but its not at all happy, won't initialise the keyboard and trackpad and won't progress beyond the login screen. Reinstalling the original 2x2GB SODIMMS and the machine boots just fine.

So I checked the new SODIMMS in another machine I have, a MacBook Pro 2012, currently running 16GB RAM. It ran just fine on the 2 x 4GB new RAM installed, so it would appear the sticks themselves are fine.

I tired to reinstall manually the EFI firmware update, by downloading it from Apple, but it won't let me install it as it says "This computer does not need this update". I checked in system profiler and it already showing MB51.007D.003.

On the purchase page for the RAM I bought, multiple customers have installed it in a late 2008 MacBook without any snags whatsoever it seems.

So what am I doing wrong? I tried multiple PVRAM resets and SMC resets and it made no difference. Thanks in advance.