I took your advice and contacted Airhelp. This is the reply I received:
"We have conducted the necessary investigation in relation to your feedback and found out that the TK1986 London(Heathrow) /Istanbul flight dated 16th March, 2019 was delayed for 3 hours 4 minutes due to technical reason (2 hours 9 minutes) and weather conditions at the airport of arrival (55 minutes). "
Therefore my claim was rejected.
Many thanks for your comments.