Thanks for replies. Have just got guide prices to help with my decision. 0.7 box profile with anti condensation backing was base price on 9×3 timbers. Fibre cement on 12×3 timbers £3500 extra plus £500 extra erection cost. 30mm composite £10000 extra. I was originally under the impression that fibre cement and box profile were similar cost but the tight budget has swayed me towards the box profile