Can't help thinking if she's full of Horse poo and grass then she won't have room for dinner LOL
Most dog food manufacturers put WAY too much on the recommended amounts!!!! I would think a whole tin of dog food and equal mixer biscuit would be far more than a small terrier like a Patterdale needs! As a comparison my full grown Bedlingtons (weight 10k) each get about 100 grams of kibble daily!
I don't know where you got the idea that all terriers are fussy eaters

I have terriers now and have lived with them in the past and none were fussy eaters
However, most young dogs seem to go through a 'fussy' stage at some point, and this is where owners come unstuck! I've had it with our own dogs. As the youngster reaches the end of their growth and begins to reach adult(ish) size their growth slows down, this is the time when they naturally need a little less food ;) but if the owner hasn't noticed or isn't aware of it then they think the dog is bored with their food and either change the food or start trying to 'tempt' the dog with niceties ;) and you know where this is going.....
Like you, the first time I experienced it, I changed the food/s, then started feeding lovelies on top, then only feeding the lovelies, and the dog appeared to eat nothing for over a month! Vet checks found nothing wrong, but we had the same thing as you, got to put on a little in x days etc.
What I noticed was that the dog would eat anything we were eating, crisps, biccies, meat etc. but not if we put it in his bowl ;) So I bit the bullet, decided what food
I wanted him to eat, and starved him for 2 days, no food, no titbits, no horse/cat poo, nada! then the first day of feeding I gave him just a small portion (about 1/5th) of what was supposed to be his ration. He had 10 mins to eat it. First day he ate 3 bits of kibble

but then he got nothing till the next day, when he finished the small portion (about 1/5th) then the next day he was given about 1/4 and ate the lot, next day he got 1/3rd and finished that, and so on, he never ate a whole ration, it was more than he needed, but once he was a good weight he had about 3/4 of what was on the box, which was right for him. He never looked back!
These days I also make the dogs work for their dinner, they have to wait by the front door and then run through the house when I call them - great fun and they have good recalls and good restraint (wait!)
If the Vet can't find anything wrong, then she's likely got you trained, something a terrier will find fairly easy to do as they're very bright and good at working things out!