Howdy again Dizzy

yes i am quite certain i shall be going for the 2666mhz kit. what i wonder though.. what is it about the SK Hynix ram that seems to work beautifuly on cod? and yet the other ram stick wont even run cod?
Cool, before you buy it, check it against your motherboard's QVL list, check online to get that information as new a BIOS update might have added support for more RAM vendors.
It worrys me that if i was to buy the new ram im looking at. the kit we discussed.. and its not Hynix i will face the same problem... im hoping im totaly missing something here though lol
I very much doubt that the issue you're having with your RAM has anything to do with SK Hynix or Samsung, I think that 1 stick of your RAM is supported by your mobo and the other isn't, that's probably why Corsair couldn't find an isssue when they tested it and that's why it is important that you get RAM that's is supported by your mobo, which is why I keep talking about the QVL.
The QVL as I think I have mentioned above is the Qualified Vendor List, this is a list of all of the different RAM modules that have both been tested to run on your mobo and have got support within the BIOS to be able to correctly configure your RAM timings when you run XMP profiles, cheaper RAM doesn't support this as it is assumed that people buying it are not interested in running their hardware at maximum performance.
This is the part number for the 2666 RAM: CMK8GX4M2A2666C16, your mobo comes in 3 versions, B450M Mortar, B450M Mortar Max and B450M Mortar Titanium, assuming you have the 1st of the 3 I checked on the MSI website but couldn't find a QVL there for your board, I have never seen this before.
I checked on the Corsair website and found
this RAM using their compatibility checker, this is what I would get and it works out £10 cheaper too, the only downside might be the colour but in my opinion that ram on a black mobo would look awesome, the red pops.
That's great news, be sure to like the posts that you have found most helpful.
In regards to the XMP i was brousing the bios and i have a setting (U-XMP) but it says not supported.. is this because the ram is budget and does not allow XMP?
I think so, they can make the RAM cheaper by not adding features that people buying on a budget usually need, such as XMP or overclocking support.