Thanks for the replies to my scenario. Now any one got a favourite way of doing it without having to climb over the load or open the curtains. 
Edit:
Think I’ve cracked it: Right its ■■■■■■■ down with rain and load is dry palletised goods (below 400kg per pallet) that can’t get wet.
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Release curtain straps and slacken off both ends on the pole tensioners
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Stand between curtain and trailer so internal straps can be slid along to strapping position, at this point you will know if your in the right position as your lower back and legs will be getting very moist.
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Start securing straps being carefull to avoid lashing in the same place as the curtain strap wants to go (nice if everything looks straight)
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After securing internal straps slide to outside of curtain retension poles and secure curtain external straps and repeat for other side.
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Jump in cab attempt to dry off, pour tea then depart for destination.
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Realise the internal straps are now probably all looser due to tension of external ones but safe in the knowledge your safe from any VOSA activity.
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Don’t forget to repeat this operation in reverse when arriving on delivery bay.
Hope that covers it but any better suggestions would be great.