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I would have to disagree I'm afraid. The fact that the tube is now sitting about 10 mm off on the two axis that I can see means that there was either a fair amount of tension in the built frame to start with, or it has been bent out by riding whilst it's been cracking/cracked. That means tubes are bent. How are you going to straighten these tubes? How are you going to tell which ones are bent and in what plains? Surface plate is best bet to find out how it's bent, but bending it is really a no no in my opinion. Low end steel, clearly full of rust, how do you know that you won't either snap the tube in the process of bending it, or worse weaken the tube whilst bending it, saving it's failure for a later date.
Also, that tube is torn, so it's a new tube, so £30 just for a tube, and the remnants of the tube will have to be removed from the Bb shell and down tube as well as top tube and seatstays.
And..... You'll need to paint afterwards. So all in, if you find someone to fix it and a non shit way for less than £200 I'd be surprised.
I paid £40 to get a small fatigue crack welded on an alu frame so for a steel frame like that, easy to reinforce it and weld it in few minutes.