Thanks so much for your prompt reply, it makes such a difference when the designers of plugins take an active role like this.
Again, I may not have explained myself perfectly because I'm pretty sure I don't want to leave the main content block. There is a white border between the white content area and the coloured background - and it was the margin of the main content area that I want(ed) it aligned with. Essentially, like this.
Like I mentioned before, the best answer I've found is to use a plugin that adds columns to the page and then place the gallery inside one of the columns. Then by adjusting the size of the images to be exactly the same width as the column it all lines up equally and properly. I'm using WP Easy Columns if someone is interested in doing the same.
So I think I've found an answer to my problem, and it's nice and simple too ;)