Renee, that's what we've been going through and I rang tresillian and this is what they said about going to one sleep:
Firstly they will vary and some days have one sleep somedays' two. Transition age is 14-16 mnths generally when this happens.
On one sleep day try keeping bub up until 12.30 and then down for nap (preferably until 2.30pm) they will have lunch first. If they can't wait that long, keep them up and at 10.30am big morning tea, then bed at 11am, lunch about 1pm.
I am finding that the last two mornings coco's gone down at 10am (after a snack) and up about 11.30/11.45. I then put her down at 2pm for her half hour time out (no sleep though).
Good luck xo