I wrote to David for some assistance with scaling images, and he was very helpful indeed... I found it hard to figure out how to scale an image by a pixel amount in one axis, and allowing GIMP to scale the other axis accordingly to maintain correct aspect ratio. This is apparent when scaling by a percentage, but not so apparent when scaling by a pixel amount.
From within GIMP, you can scale an image using this dialogue box which gives you the option of % or px and both can be linked to keep aspect ratio:
DBP gives this screen for these options:
To scale an image by a pixel amount in one axis and let GIMP do the rest, for example scale a width to 230px, you have to select Fit to "Outside" and specify a rectangle size of 230 x 1 as in the image below: