I changed over to viola from violin 6 months ago and despite learning the alto clef very quickly (and quite accurately) I feel that my sightreading (for both viola and piano) has got worse. My piano sight reading was never especially wonderful anyway but now I'm reading alto clef instead of bass clef. Has this happened to anyone else or is it just me?

Yes, I found that I initially read bass clef as alto on the piano. Lovely harmonies!
I have got better though. I try to make a concerted effort not to make any associations between the instruments and then I can become more automatic with reading. I tend to read by intervals a lot, so if I start on the correct note in the bass clef, no problems. good joke!

I switched over about 6 months ago too, and I've spent a lot of time just generally focusing on my sightreading. I haven't had the problem of reading the bass clef like the alto on the piano. All I can say is that it takes practice, buy yourself the specimin sight reading tests for viola AND the piano, and for the piano also buy improve your sightreading by Paul Harris, and just spend a LOT of time sightreading, and gradually your sightreading will improve, its just a gradual process. Also, try and join an orchestra, or form a quartet or a trio, that will also give your countless oppertunities to improve your sightreading, trust me cos the day after I started the viola my teacher put me in a string quartet and that helped me improve my sightreading in general, and my sightreading is better than I think it would of been on the viola if I hadn't joined. But you will begin to just know what celf you are playing in without thinking about it in time.