Found it unfortunately It's the A500Plus service manual I have too... 
I couldn't say therefore if C15 is different from the original A500's C15 Capacitor but the only details given in this service manual are...
C15 = MLC AXIAL Z5U .01uf
(If I recall correctly the Z5U means that this capacitor has a 50v rating)
However as you have said that your one is 0.1uf then it looks like C15 in an A500 and C15 in an A500plus are two entirely different sized components... 
If the capacitor is the Un-Polarosed type then these generally have a minimum rating of 50v (usually a lot higher), hopefully someone else can supply you with the appropriate details... sorry...
Hi,
Only Amiga, come on lets all sing it together.
smerf
