Peaches, I'll cut and paste bits of your post here as it makes it easier to comment.
"I've been trying to express but when was exclusively bf was impossible as he would be crying so much and then after 1hr + feed would want feeding again an hour later!"
I don't see a reason for expressing - it is one way of getting milk out, but in your case, the situation cried out for someone to observe you feed and find out more about what was going on. Bf, expressing, formula feeding and feeding with EBM is the hardest way to feed a baby - but it often happens that mothers find themselves on this treadmill and it is hardly ever necessary.
" I could only get 20ml or so out and found it adding to the stress of situ greatly."
Precisely. That is what happens
"...stockpile of ebm for when my milk does go so at least he'll have some bm for a few more weeks- is still taking me 3 sessions to get enough for 1 feed though."
This is too much like hard work!
"At bf cafe I told bfc that he kept pulling away and fidgeting during feed and she said was wind! I always thought it was due to him not getting enough milk maybe due to poor latch but was confused by the face full of milk/ sick that would still get at end of a feed. Also he appeared to feed properly with swallows, gulps etc..."
Expertise varies hugely when it comes to assessing a baby's feeding. Can you get a second opinion?
"Re the doctors and hv advice to top up with ff was taken as he didn't put more than few grams on per week over course of 3 weeks- do you think was taken too quickly?"
I don't know - it sounds to me as if they did not know how to improve your bf
"I have tried to put him back on breast after ff and he gladly takes it but is still screamy/pukey and I worry it's making any overfeeding worse."
Again, someone needs to see what is happening.
No substitute for this, I think...