Not sure which of us is further south but I am DEFINITELY! a lot further south than the inner hebrides (#Jersey).
Having said that, if you can fix the wind problem, you get a lot more daylight than we do in the summer which would compensate a bit. I think heat and wind are probably the issues - and that polytunnel sounds like just the job! I've been offering to get up to N Uist to help a mate install one just as soon as he gets round to buying it.