In reply to 19G:
If you find any with all of those attributes I want a set.
There are stretchy mountaineering trousers and there are non stretchy convertible trousers. The combination of stretch and convertible is quite rare.
Decathlon used to make them. Did not think they would last long and you would need to be tall and thin to wear them.
Nearest that might fit the bill are these Montane ones below.
http://www.gooutdoors.co.uk/terra-stretch-converts-p289461
Hitch n Hike had them in last year as well.
I have the lighter weight normal non stretch Montane converts which are fine for summer. In the stretch converts however, I found the fit was very poor and although stretchy fabric they were constricting at the knee. Didn't buy them as I would have had to go up a size for freedom of movement.
I do use North Face converts which are non stretchy but very tough material and fine for rock climbing. Not sure if this model.
http://www.thenorthface.co.uk/tnf-uk-en/p96627.html?_ignore_category=1
There are some very good non convertible trousers out there. Rab, Montane being my favourite brands. Would recommend trying them on as fit is crucial.
Good hunting
Post edited at 23:28