Thanks Anne,
Yep, I have a few other pics of this. I chose this one mainly because the distortion is so great as an extreme example of several photos I need to fix.

Phil: Good idea... I might trial one or 2 of the commercial apps when I can find the time.

I realized that I've had GIMP installed (on my Linux box) for some time. After just a little scratching around I found that GIMP does have a perspective tool, and I've been able to achieve results very similar to yours, Andrew - Thanks for that!
I'll be sure in future, too, to consider this correction and necessary cropping when composing similar shots.

To be honest, before researching this today I did not really know the cause of this distortion, or that this perspective distortion was quite different from the lens (barrel/pin-cushion) distortion which I had read about.

It appears that there is a plugin for GIMP to provide lens distortion correction.... Further research required...
So, at least for now, I'll stick with using ViewNX2, and GIMP when more tweaking is required.

Cheers,
Matt