De-icing has been hit and miss at LTN for ages....A combination of at least three different companies involved, certain staff who probably haven't dealt with a serious snow/ice event before and poor co-ordination.
The main reason the TUI departure to ACE got away on Dec 10th, was because TUI are the only airline customer using the de-icing rig of Airline Services. So, efficient de-icing was carried out with an eye on the planned runway re-opening time after snow clearance. If this hadn't happened, the crew would have gone out of hours!