Andy, I'd suggest using a clamp-on style, shift-lever mounting bracket instead of doing any type of welding.
You should be able to find a clamp style bracket for sale in the UK.
Shifterkarts are called Superkarts in the UK...
You could take a look through this google search link to hopefully find a shop (in the UK) that sells the mounting bracket you need:
http://www.google.com/search?source...96f4bfc11aeef5&bpcl=35243188&biw=1024&bih=556
If you cant find the bracket listed on any of those websites, call or email some of those shops and ask if they can help you find one. It'll be strong, 'look clean', be easy to instal and probably wont cost very much...