Yes, yes, yes, slow and steady would be the 'best' way to do it all. 'All you need to do is eat less, move more.'
There are millions of pounds being spent on obesity research in this country. I used to be one of the clinical researchers in this area, and, you know, it just isn't as easy as some people want to make it out to be. (I've linked to research from Cambridge and Oxford profs before, but there are always people on MN who know better!)
In holly's case, I can completely see why a short, sharp shock would be the best thing right now. There is a limited window for IVF and doing something in 6 months would be far, far better in terms of success rates than in 5 years time.
Lots of people put weight on after losing it. That comes down to genetics affecting how different people's appetites work, the social-environmental pressures of living in a obesogenic world, your hypothalamus trying to get you back to your previous weight in preparation for the famine that (now, for us) never comes, the psychological reasons that contributed to someone overeating before.
Research shows that there is very little difference in the long term outcomes comparing very low calorie diets and more traditional low calorie diets. It basically all comes down to how that individual changes their life and how they remember that they can never take their eye off the ball, or it will go back on.