Mmmmmm, it's been great to spark a good response and I find on this particular forum I'm far more likely to get intelligent and thoughtful responses from people who really know their stuff than I can from any other forum. Every response has been interesting to read so thanks to you all. Am I any nearer a solution? No, but I have been able to expand my knowledge and understanding and that's never a bad thing. So, what have I discovered?
1) There's no clearly defined right or wrong to this question. Whether an amp has better distortions or not depends on the amp you're using and the same is true of pedals.
2) Although there haven't been many concrete suggestions on amps I could look at in my budget, I get the impression that I should keep a very open mind. The Marshall JVM 410H remains an option, as does the Blackstar HT Stage 100. To those two I am inclined to add a Marshall DSL50/100, Laney TT-50, Laney VH100 and Laney GH100.
3) As tastes change I should perhaps look for an amp that does the ranges of distortion I want but also has a decent clean channel to use distortion pedals on if that's the sound I decide I'm after.