I presume this is for growing tomatoes or suchlike during the summer? If so I dont think fastening it to the guttering with clothes pegs is a good idea. The slightest bit of wind (and lets remember we have in effect, a 20 foot by 7 foot parachute here!)
will rip it and probably the guttering down!
Better to fasten a bit of timber below the guttering to the wall after wrapping the plastic sheet around it a couple of times and making a polythene curtain.
Although more costly and time consuming, I think a basic wooden framework could be knocked up from below the guttering with something like slaters laths and covered with whatever you fancy, or can afford. Remember you have to be able to access this structure some way so either roll up or hinged sections or even lift out sections are a must.
Good luck with the project and let's see a piccie of it when it's finished!
