Thanks for all your replies and I'm glad I'm not alone
Last night I finished doing the vertical stitches and was able to remove all the safety pins.
I decided to investigate why my walking foot was slipping so much. 3 things are wrong with it.
1: The teeth seem to have worn down on it. Not sure how to fix that. Maybe some delicate filing.
2: The part with the plastic teeth wasn't pushing through the metal foot evenly and not very far. I've since straightened that and bent its arm a bit to put more pressure on the fabric it's pulling.
3: The teeth on the foot and the teeth on the machines dog feeds don't line up and so I can see it moving slightly to the side when the fabric is clamped between them.
In trying to remedy issue 2 I used my default precision bending tool, the screwdriver. First thing to pop off whilst bending and twisting things was the plastic housing for the foot. No biggie. It pops back on. Second thing to pop off was the actual presser foot that swivels independently from the rest of the foot assembly
That took some brute force and prying things apart to get it back in but all is well. It seems a bit looser than before but I'll keep and eye on it. My vice will sort that if it becomes a problem. There's nothing quite like precision engineering