I am guessing the bend was caused by someone stepping or sitting on the tiller while it was mounted in the rudder stock. The bend is then probably close to the rudder stock. It may be easier to first try to bend it back up while it is in the rudder stock. If that fails or if you need to apply a too much force it is time to think about another strategy.
A vise or better, a Black&Decker workmate may help. Use some old rags around the tiller to avoid scratching it. Filling the pipe with sand is a way of preventing a pipe from collapsing when bending it a tight curve. I don’t think it is necessary in this case but it wouldn’t harm either. You do need corks in the ends to put some pressure on the sand though. Tape the corks so they can’t pop out while you’re bending. And also make sure there are no voids in the sand filling.
If all else fails heat may help. If nothing else, heat takes bending strains from Aluminium. You don’t need a welding torch for de-straining Alu, you can do it on a normal kitchen stove. It goes quicker if you take the cooking burner off and light the gas without it. This way you will probably get a nice pointy blue flame like a Bunsen burner, that will heat the Alu in seconds. But please remember to take al plastic parts off before heating it. When it gets dark red hot all strains leave the material as by magic. But lets hope you don’t need to heat it up.
And finally, Aluminium pipe is also available in building/hardware/DIY shops and I guess at much better prices as the original sold by our chandlers. (For example Pinell & Bax sell it by the meter). Maybe you can find some identical pipe in a hardware shop and move all parts from the old to the new tiller. Make sure the pipe has walls as thick as the original pipe to ensure it will be strong enough. DIY shops often sell thin walled Alu- pipe which is good for hanging curtains from but definitely not strong enough for use as a tiller. To finish it, carefully rub it with 1000 or 1200 sandpaper until it lost all its gloss. Then leave it a couple of days to allow it to receive a nice patina (Alu- oxide coating). Maybe it is not as beautiful as the elox process Seldén/Proctor uses, but it will work. If not, use some varnish on it after rubbing it with sandpaper.