Finding some leftover cooked potatoes today in the fridge, I had a brainwave and decided samosas were in order, so I revisited this great recipe. The vegetarian version of filling recipe from the OP worked well with 200g cooked potato, quarter of a mug full of frozen peas and the same amount of finely chopped carrots. 750g of potato, as mentioned in the OP, seems way too much for those spice quantities IMHO.
Well the samosas were great and because I used only half the filling up, I'll be cooking more tomorrow. Just a simple classic indian snack and having scoffed a few for lunch I wonder why I don't cook these more often. I guess they took an hour, start to finish. Not much effort for a good result.