I only have a copy of the A500+ manual but in there C15 is a 0.01uF (10nF) capacitor, non polarised (please re-check your manual and advise if yours says different. From here on I will assume C15 is the same in your machine as the A500+). As JimS has already said, it's a decoupling capacitor and is not critical to the operation of U15. You could probably get away without fitting a replacement, although obviously this isn't ideal.
I checked the parts list and it looks like we're talking about a through-hole axial component, right?
Please see my info attached.
The capacitor is described as a Z5U device in the parts list. That means it's designed to operate between +10 to +85 degrees C and has a tolerance of -56% to +22%.
Get yourself a replacement ceramic capacitor (MLC means Multi-Layer Ceramic) that matches those specs (easy peasy) and you'll be just fine!
If you want further help choosing a capacitor let me know and I'll suggest some replacements from Farnell for you. In this case please photograph the component for me so that I can suggest one that will match the footprint of the original component. I'd dig out my A500+ and find it myself but I've just moved and my A500 machines are both still at my old house!
Cheers!
AH