I'm not sure if it was done on the usual basis Jen.
"The poll allows for big variations, however, and suggests that the Tories could get as many as 345 seats on a good night, 15 more than at present, and as few as 274 seats on a bad night."
It looks like the modelling is controversial (especially with Tories I would think) but they used it during the referendum and it always had leave ahead.
Ah - found it.
The model is based on 50,000 interviews over the course of a week, with voters from a panel brought together by YouGov.
This, apparently allows a different and more complex assessment - the details are in the article.