I should have mentioned that the frame gets bent down. The side I knocked flat was done while pushing the rear of the frame upwards. While doing that, the frame "wants" to go back to being flat and in my case it was quite easy to knock the bulge out.
I found some pics of my frame.
Here's the slightly bulged side:
And, hard to see, but the corresponding slightly bashed upwards lower rail.
And the much more seriously damaged side featuring a back in the day repair.
Same side from the inside.
The bad side repaired.
Mart.