Loads of good alu weldera in the uk , aerospace or marine places are also good to try.
You may not find those recomended welding rods in the uk but you can use a castolin tig-tectic 21 or 23 rod that will work well:
nimubus welding guide:
If you are unsure fo the weld fatiuge and heat treadment. try searching online. it will probably tell you to put in the oven for 6 hours at 200 then 3 horus at 180 then leave for a week or two to get full crystallation.
Also you could have teh bb it reamed and then sleaved with a bonded steel sleve afterwards as the steal sleve with spread the load. argos cycles do this.