Thank you very much for the answers.
Arf, unfortunately, I don't have anything to heat-up the tubes, like a blowtorch (I just have my lighter, but I don't think it will be sufficient
) But thank you, it could have been an easy way to get rid of the problem.
I chose the option to enlarge the holes. Now they are oval, but at least the roll-bar fits. Actually, it doesn't move even move left and right, as the bolts are holding it on the "new" outside edges of the holes, on each side. I'm tempted to keep it like that, but I will try to weld up the edges and grind to go back to round holes as Kev adviced.
Thank you very much for the help.