I've just been to B&Q at Hermiston Gait, Edinburgh. They were selling decking boards 25mm x 120mm x 2400mm for £2.98. I assume that it's nationwide.
There is 15% off today so that brings the price down to £2.53.
I know that B&Q were doing decking for £1.98 last month (I bought lots!) but this looks better quality. I wish I'd waited a paid the extra 55p!