I'm having the same problem, what I did was selotape the spotlight in and then silicone half of it until it sets, then I removed the selotape and siliconed the rest. Not ideal but it works.
Tynesider
October 2010
If the hole is not perfectly circular try rotating the fitting to see if it can be supported better elsewhere.
Otherwise you could glue a small piece of plasterboard to the inside of the aperture.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lighting.