Thanks for the replies. Mark, I hadn't noticed fresnel in my shader tree before, but I then realised how to display it thanks to your illustration. I set the fresnel to 26% (simply because that's what the picture shows) in both reflection and transparency. Then, rather than using the refraction of the clear glass preset (which I did try with with fresnel), I set it to 1% instead. This is the result. I think it looks considerably more realistic than my last effort. Do you think I should decrease the refraction more and/or increase the fresnel for even less distortion? Cheers.

PS: This render only took 7 minutes with lower settings and a smaller image. I've since learnt to leave the big render until I've got the shader settings right. :)