Right, So my blog currently has a 50/50 UK/US traffic split and I have a few perfect opportunities to promote some amazon products on there. However I have heard something about if I have a US affiliate id I wont get paid for any customers buying from the UK Is this true? and if so what can I do to stop it?
That's correct. You need to sign up to both the US and UK Associates program to get paid commissions from both. The links will have different IDs in too. If not then when someone goes to say the UK site you won't benefit from any sales made.
You should try and redirect your US visitors to a new US site targetted site. Visit Amazon.com if your in the UK and you get a notice saying "Do you want to shop at Amazon.co.uk?" You can do this via geotargetting.
Geo-Target is a right way to go. You can use the script at this site http://petewilliams.info/blog/2009/07/javascript-amazon-associate-link-localiser to easily do the Geo-Targeting. This script uses Google API Javascript to get the user location and then replaces your Associate ID in the Amazon URL so that the user gets redirected to his relevant Amazon store. You will need to signup to all Amazon sites that you want to use on your site.
It doesn't appear that it's working right now. I clicked on the example link on his page and it didn't send me to amazon.com, it sent me to amazon.co.uk but I'm in the US...
You can find a good, free database here: http://www.maxmind.com/ - this is what I use You do have to code it yourself tho... it is a list of IP ranges and their respective countries/towns