How to stretch image section

[Blighty]

To stretch the right hand side:

1. Make a selection on the right hand side.
Click inside the selection to finalise (Free Select tool - double click inside)

2. Edit > Copy or Ctrl-C

3. Edit > Paste or Ctrl-V
You get a Floating Selection in the layers dialogue. Right click and select To New Layer

4. Image > Canvas Size and increase the width.

5. Select the Scale Tool and scale the new layer. "Keep aspect" option in the Tool Options must be off.

Note:
Static dotted line is a layer boundary.
Moving dotted line (marching ants) is a selection. Select > None to unselect.

To flip and join the right hand side.
Do steps 1 to 4 above.
Then use the Flip Tool to flip the new layer horizontally. Use the Move tool to move the layer into position.