I’ve managed to pull the head off one of the 100nF ceramic caps (labelled 104) for my filter board already. Great start…
Anyway, I have a mixed bag of ceramic capacitors in my collection. And I have multimeter. Is there anyway to tell which will be a suitable replacement or do I give up for the evening and head to my electronic parts store in the morning? I saw one that says ‘104M’ but not exactly sure if that’s the correct replacement.
Where was the 100nF cap located (index on the board?) there’s a good chance it’s a decoupling cap and you can do very well without it.
I’m missing the one at c23. Is this a decoupling cap? And what is a decoupling cap? 
a decoupling cap reduces power supply noise at the power input pin of ICs and is usually right next to that pin, between V and gnd.
Yeah, you can go without it.
104M translates to:
104 = 10 with 4 zeros after it to give picofarads = 100,000pF = 100nF
The M = 20% tolerance
So your 104M is ok to use.