Ok, I have sorted this. The problem is that we have all thought that the theme would take resized photos and post them. Provided the height of the photo is the same as the ‘Slider Height’ then we shouldn’t have to worry about the width, right? WRONG
The slider will only (possibly, I have not tried different sizes) accept a photo that is 1500 (width) x 350 (height). The chances are that if you resize a photo to this size then you will have to ‘un-tick’ any aspect locking and the result will be that this will distort the photo.
The way around this is to start with a blank canvass with the same colour as the background colour of your site and set to 1500×350. Then past your photo on to it and then save the canvas as a jpg. You can then use this photo for the slider.
This nugget of info would have saved me hours. I hope this helps someone else.