In reply to miggins1610:
> Also on the down jacket front, i tend to be a medium but have been advised in Cotswold outdoors to go for a small so less air can get in and affect the insulation. Is that right?
A small what? There is so much variation between brands - and bodies - until you're getting down to specific models it doesn't mean much. Also, while a trim close fit does keep warm air in better, around your body, if you size down and it's too short in the arms or too short in the body those bits will be exposed and defeat the purpose of a warm jacket.
Air 'getting in and affecting the insulation' is not really a thing, btw. It's really quite bad advice from a shop.
Khumbu in March you don't really need a hardshell at all - do you really think it will rain? A good windshell or hooded softshell will probably be fine. NZ SI, on the other hand...