I was thinking about fabricating axle brackets out of 3mm steel using 2.5mm arc welding rods and butt welding - are there any legal issues with using this method (e.g SVA?) rather than using 50mm RHS?

I've destruction tested some sample welds and destroyed everything on my work bench other than the welded pieces so the process appears OK - anything I need to consider?

No particular SVA requirements, just needs to be 'strong enough' in the testers opinion.

Saying that the tester at my SVA test didn't seem to pay any particular attention to the back axle. Just a quick glance.

If you can make a decent job of it - it should be fine.

When Welding 3mm to 1.5mm can be difficult to produce a decent strong weld.