HP20,
I passed Stage 3 in the middle of January, and was told they were allocating spaces at that time for Course 219 (April start) but I'd be on 220 due to my notice period. After my CAA medical (early March) I gave HR a quick call to check I was still on 220 and everything was ok - it was. In May we found out the course date had been changed a bit, but apart from that, I've been pencilled in for 220 Aerodrome since I passed really!
My best tip is to give Nikki a call at HR - she can tell you which course they're filling up places on now, and note down what you want and when you'd like to start.