Thanks Memoria damnata, I managed to get the edge of the blade square again, by grinding it with an angle grinder edge on at 90 degrees while the blade was spun slowly in a pillar drill.
The blades cut really well after this and more importantly they cut straight.
I will dress the blades while cutting, every metre or two of cut and see if I can keep them cutting true.
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Some info from Marcrist:
Dress the blade by passing it through the dressing stone 10 times. Do it dry.
The stone needs to be abrasive and soft. Breeze block can be used or old soft cermaic tile offcuts.
They are replacing the blade and maybe if I dress once every other cut, it might stay cutting straight.