What capability do you have equipment-wise to do this welding Cathal?? I ask because you are telling us your are new to Aluminium so I want to make sure I inform you properly.
Well as far as tig welding goes you could experiment with 4043, 4643, 5356 as fillers (perhaps some others in the 4xxx,5xxx range). The most important consideration here is that the weld area is completely clean and free from any paint, oils, dust, abrasives. Acetone is a very good solvent for cleaning aluminum as it leaves no residue. For preheat yes oxy-acetlyene....just gentle warming to ensure a good weld... you will see a very very light gold tint appear when the bare aluminum is at optimum temp (does not take long at all). Do the preheat after you have fitted and tacked the two pieces together.
Now its possible you could recover your tonality with proper post weld heat treatment of the material to restore it to its original t6 condition...this would require proffessional help and equipment for the optimum results....and probably would not be worth the expense when you could simply get another piece of material in the length you need.
If nothing else here you might pick up some esperience and feel more comfortable working with aluminum in a welding situation if you do a bit of "experimenting" ...just don't experiment on costly pieces until you are comfortable with the projected outcome.