My grateful thanks to Adrian (Adrian's Images) for suggesting that holding the shift key when painting would give straight lines. Working on the 'rainbow' layer I erased the previous 'rainbow' and chose a different brush, Simple Watercolour. It was then an easy job to choose the size and colour, tap where I wanted to start a line, hold the shift key and tap where I wanted the end of the line. Bingo, a straight line is automatically drawn. Keep changing the colour and adding new lines from the Sun to the title:
I am much happier with this result. I think I will have no hesitation in purchasing the final version of Serif Affinity Photo when it becomes available in the Mac App store. It will be a very worthy replacement for my old copy of Photoshop Elements 10.