Hello, I was at my local Independent VW specialist today having our other car (Up) serviced and mentioned this to him.
He told me that the had a few cars in with the heater matrix becoming blocked. However, this is not caused by the bag in the tank bursting. Apparently, in his words, there is a scale forms similair to limescale that forms on a kettle and he has seen this in both the matrix and on the bag with the bag remaining intact.
Their fix is to flush the matrix and in the main this is succesful but for some only lasts one year then it needs to be done again. Apparently VW had invested in equipment that is used to flush the matrix that neutralises the contents of the bag but have now withdrawn it as it is not believed to be the culprit.
His advice to me was there is not much you can do to prevent this and to leave the bag alone as there does not seem to be a pattern as to what causes this.
Just to clarify the 2 guys who own the garage are ex main dealer trained mechanics and used to work for Audi and VW and have recent experience within the dealership of this issue.
I hope that helps and doesn't muddy the waters too much.