I was lucky to have a good dietician I needed it as DD had failure to thrive for a while and did not gain a single gram for over 6 months, I did see a couple of dodgy one though in regards to getting her off the tube and I know what you mean by the common sense advice

I did put formula down the tube after I stopped expressing for a while although that was when she was having nil by mouth. Once she started eating and required extra calories but not nutrients I got a tin of polyjoule from the chemist, you don't need a prescription you just ask for it over the medicine counter, it is tasteless and you can add it to their weetbix, drink, yoghurt, anything really and it adds in those extra calories.