I am currently training at an altitude of around 6000ft will I notice the difference when I return to lower levels? I will have completed 3 weeks at this altitude before I return to 'normal'! I'm currently doing 3 weight training sessions (1 a day) followed by a CV session then three more weights session in that cycle. Will this make a difference if I'm training at around 80% my Max HR for around 40 mins during my CV sessions? This is no more than I usually train when I am back home in the UK.

of course you'll notice a difference. You'll be able to go longer and a greater levels all the way around.