Thanks so much for the reply Kleinstein and so soon too! I guess to find the value I will have to take the caps off the board and measure them? Is that right or are there some pitfalls to this idea?
I'm going to say straight away I probably have a flawed understanding of how this circuit works. What I think happens is the resistors control the voltage out ie 20v in = 1v out, and the caps I presumed were there to smooth things out. Which is why I am finding doubling the resistance counter intuitive. If I double the resistance would it not be the case that 20v in becomes 0.5v out. I guess i'm confused.
Forgot to say I also have the book electronics for dummies which I feel I may have to read over again
w2aew - not silly at all, and to answer your question: I sometimes connect to car engine sensors using 3m long bnc to 4mm banana jack leads connected to back probes or at push insulation piercing probes. I could use a normal 10x scope probe with a bnc to banana converter but it all just seems to get a bit clunky. An attenuator at the scope would solve this.
Some may be wondering why don't I just buy one? Well I wouldn't learn anything, it seems wasteful if it can be modified for less expense, and I find this sort of stuff fun to do. The learning bit is the most important though.