Actually, I have been using "Default App" to set the defaults, which has always worked fine up to now. What has changed since installing Avast is that the defaults are cleared when the phone restarts now. I have also tried setting them manually (by clearing them and then choosing a default when I load the relevant application), but the same thing happens.
You make an interesting point that I hadn't realised before - that those few defaults that are not cleared are for apps on the internal storage. All the apps on the SD card have the defaults cleared. Sadly, Sony are not very generous with internal space, and I am unable to move them all back to the internal storage.
Would be great to root my phone, however I am not particularly technical, and understand that the rooting process is fraught with risks.....