Thank you all for your comments. I think the Jeans Machine was designed to be heavy-duty. Unfortunately, I have tried it and I'm afraid I didn't like it, but I know some people swear by it.
Originally I thought I would be looking at the ones that cross between domestic and industrial but the very HD ones seem to be designed to do a lot of leatherwork but I can't see me going in that direction. Added to that they are a whole new learning curve.
The two I am currently looking at are the Janome HD2200 and the Elna eXperience 540S. Has anyone used either of these? They both seem to work with rotary rather than oscillating bobbins which seems to be better, when using fewer stitches, at keeping the noise down. They have fewer stitches and go through layers of seams, etc., easily. Of course, I can't get out to road-test one so I am picking your brains 
HD machines are generally metal inside rather than plastic (so heavier). I have a machine that does have multiple stitches but I use very few of them. I may in the future but currently, I need a workhorse rather than the bells and whistle type.