Good point, Jon. Equally, there's no mileage in trying to define 'English trad music'; though some general characteristics can be identified in regional styles.

The MS tunebook issue having been dealt with (they are full of turns, rolls and mordants) I see no reason why decorative techniques nowadays associated mainly with Irish styles shouldn't be used once again in English contexts.

I learned fiddle largely in the 'Irish' sessions around Sheffield in the '80s (that's all there was at the time) and still have a lot of Irish tunes I can fall back on, but I've concentrated on English repertoire for a long time now. I use more decoration than many 'English' players, but nobody has ever remarked adversely upon that. It ought to be used with discretion, though, and mainly for punctuation and emphasis; some players in the 'modern' Irish style use so much that the melody disappears almost entirely behind the 'aren't-I-clever' frills, and it would be quite impossible to dance to.

One English fiddler who started out via the (Liverpool) Irish tradition and has subsequently devoted a lot of time to exploring the background is Gina LeFaux. Her thesis would be, I think, that 'Irish' ornamentation is basically a survival of the normal Baroque style, which went out of fashion in England, whereas in Ireland it didn't. Of course there isn't an 'Irish' style as such (the Kerry and Donegal traditions, for example, are wildly different from each other); what you tend to hear in 'Irish' sessions is an homogenised, 'mix and match' thing. Essentially a modern construct. Regional styles are more distinctive and a lot more interesting once you start exploring them, and that's true of the whole of Britain and Ireland.

There will always be people who will say 'What does it matter' or 'Just enjoy the music and don't think about it'. That's fine for those who have limited horizons and small imaginations. It's bad advice for the intelligent and enquiring mind, though, which seeks to understand as well as to enjoy.