Its a difficult one cause we all lose weight differently. I was about 14st when I bought mine 18 months ago, and I was just short of 18st at my biggest. To be honest, it fit perfectly all the way between those weights- at first I used the tighter hook and eye, now I'm on the looser one. Admittedly, it is maybe a little more uncomfortable the bigger I was (I had a lot more squeezing in to do) but still very wearable. At my sisters wedding 3 weeks ago I wore it from 11am to 3am and it was so comfy I forgot I even had it on!
If it was me, I would invest in one now while they're on sale and it'll last you ages, but I suppose it depends on how much use you'll get out of it. I tend to wear mine a lot, so £45 seems a bargain but might be a bit much to fork out for occasional use.