Thank you all for your kind assistance. I have been trying different things today including flattening the blades then getting the two flat ends in the same plane, cleaning ends with thinners as well as slightly sanding them. Sorry didn't get on to well with the chin thing. It doesn't seem that the blade is slipping but that after doing a pre-tension / pre- stretch whatever it is called although the blade will tension on and off and stay at the same tension, for me at least, the spiral blades need a bit more tension to go where I want them to and not want to wander. For someone with more experience the normal tension would probably be OK.
The only way I found to get more tension with consistency may sound back to front and after reading the message from Grizz may be more wrong than I know. After normal tensioning the spiral blades do not need a lot of extra tensioning to make a big difference for me. What I tried today was to tension as normal then release tension and the top clamp and move the back knob one and a bit, of the finger notches, to the right - yes to the right opposite to the direction normally used to increase tension - then re-clamp the top clamp and re-tension. This lowered the top arm maybe 1/16" further down the blade and, with the lever giving the same tension movement on the top clamp, effectively increased the tension enough for me.