Thanks to all for the very interesting replies!
I should have explained that the "trick" for straightening after unrolling was simply to crush flat both ends of the tube, firmly grip one end in a heavy bench vice and a large mole wrench at right angles to the pipe at the other... then give the inside (from the vice end) face of the wrench a really good whack with a hammer! Apparently it appears to work like magic every time?!
Admittedly, having only ordered a one meter length, I've not got an awful lot of waste material to play with - which is another reason for deciding to just do the best I can with what I have!
(part of image taken from an unknown source, and shown here for illustrative purposes only)
The above picture was chosen for scaling the pipe run, as it is probably a finer example of the Horwich coppersmith's work than all the other seven locomotives put together. Most reveal quite a variety manufactured bends - and an awful lot more unintended ones too - so I do have a little leeway!
Fortunately, as you can (sorry, just about!) see in the drawing below, the radii at both ends are reasonably generous - all that is except the bottom curve on the left hand pipe (shown here) - just at the point where it enters the front face of the well tank beneath the running plate!
I can use a tool for that bottom one, but I do quite like the idea of using fine, dry sand inside the pipe to prevent the larger bends from squashing when worked by hand. According to a contemporary published article, the Beyer Peacock specifications called for the use of resin (although I don't recall any mention of what type) to achieve the same end.
There will be a bit of fun, and probably a fair quantity of crisp language to come later, down in the railway workshops here anyway!
Pete.