@rookiecrafter. YOU ARE A GENIUS. I unpicked a small section of the shoulder seams and threaded a length of elastic through the back part of the neckband - stitching it down at both ends - and it worked brilliantly. The T-shirt no longer reveals my bra or my cleavage.
LOVE IT. THANK YOU.
@StitchWench. I waitress in a busy Italian restaurant so comfort is a priority; and with the humidity in Sydney lately I need to be as lightly dressed as possible. In winter, though, I will definitely do something like this. Thanks for your suggestions.