Thanks again to all for the replies and suggestions. This is in progress - I tested the smaller size of the same pattern and it gaped less, but still gaped a bit - I wondered about going down another size then trying a FBA but decided it might be worth, instead, learning how a pattern is put together in the first place - it might help me spot potential problems in the future.
So, I watched some videos, took some measurements and tried some drawing. It's not quite there yet, but I think that in the piece I'm practising on I have enough scope to let out, take in, adjust a bit more until I get something I'm happy with.
I hoped to finish today because school starts tomorrow and after that I'll have less time on my hands but it hasn't worked out like that.