Also just for reference I was quoted £30 for fitting. (Did the fitting myself tho!)
One thing I did find was my windscreen seal was quite damaged on the inside and the string kept catching damaged parts and wanting to make it worse (by pulling into the nicks). What i found was if you pull directly into the car away from the windscreen the seal will happily follow, then move the string sideways (past any damage) then repeat.
Was easy once I discovered how to get past the damage!