I've had a look at the dent and think that it could be reduced without breaking the paint by using a candle, a plastic tool shaped like a candle which you tap around the edge of the dent. A hot glue gun method might work in reducing it as well. There are clips of dent removal on the net.
I can remember a post a little while ago where the frame was put between some rollers but I think the cable mounts could cause a problem.
My Granada has a 531 butted frame and had a few dents on the crossbar and they seem to be a bit prone to denting.