When someone other than a thatcher needs to get onto the thatch to do some work, it can cause damage to the thatch. Window cleaners are a prime example of someone not knowing how to position a ladder on the roof to carry out their work.
The first rule is to keep off the ridge and aprons (the ridge underneath a window). When a ridge is new it is fine to put a ladder on it, but a few years down the line the wood work becomes brittle and any pressure will just snap it. Also, if a ladder is digging into the cut of the pattern it can ruin the look of a ridge.
All ladders should lay flush with the coat work. This means keep them at the same angle as the thatch. If the ladders are at a different angle then the top will dig in and leave a dent in the wire netting, or worse still, leave a dent in the thatch.
If you use multi layered ladders then you should turn the ladder around so that the top layer is closest to the thatch. This stops the top of the ladder underneath digging in.