I am changing my cambelt next week, and reading up on how to do it Dayco recommend that you fully rotate the crankshaft pinion anti clockwise before fitting new belt.
Has any one come across this?

They are saying to fully rotate the crankshaft pinion, not the whole crankshaft.
The crankshaft pinion has a left-right play of about 5-10 degrees on the crankshaft. It is used for fine tuning the position of the camshaft vs. the crankshaft.
What they are telling you is to position the pinion in the leftmost position on that slot, for easier installation of the new belt.
Of course after you secured the crank and cam shafts in position with the locking tools and after you removed the old belt.
After you've installed the belt and properly tension it, you'll see that the crank pinion will sit perfectly centered vs. that key.
The key is there only for holding in proper position the magnetic ring, not for holding the pinion.

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