I'm not an optician and I don't know much about RAF medicals, however I am astigmatic and I did have trouble in years gone by getting a civvie class 1...
There is nothing much you can do about astigmatism, any more than you can 'cure' short sight (the two often go together by the way). 0.25 dioptres is about the minimum a high street optician can measure, so you have very little - are you sure it was only astigmatism that cuased the medical failure?
Your varying prescription can be due to measurement error and/ or age - my eyesight didn't fully stabilise til I was about 22. Your optician will do a couple of tests to establish both the severity (CYL number) and orientation (AXIS number) of astigmatism - it's worth reading up on the tests to make sure that the optician does it properly and you give consistent answers. That will give you the best chance of a 'true' reading.
There's reasonable information here:
http://en.wikipedia.org/wiki/Astigmatism to get you started on research.
Can't help much more I'm afraid - it's worth saying that as long as other aspects of your vision are OK, 0.25 astig will not prevent you from a civvie job...
Good Luck