if the 230V is a little high, you'll get a little more on the 15 V side
15V RMS is 21.2 V peak
one diode drop through the bridge, a second diode drop through D1,
and a third diode drop on the return through the bridge yields 21.2 V - 3(0.7 V) = 19.1V
you need to put a load across C5 to see anything besides the peak voltage
diode D1 and capacitor C5 act as a peak detector, so you are seing the peak voltage
how much current should the 15 V supply provide?
connect R = 15V / (expected current in amps) across C5
you might want a three terminal regulator after C5 to provide regulated 15V
as you did for the 5V supply