Hiya J1mbo,
I cannot explain it either, but after 48 hours of unticking IGMP Snooping I have not had a single dropout or buffering on any device when using Tunein, Spotify, iTunes Airplay or anything. The response from all those applications a far more responsive as well.
The setup is basically a Draytek 2865Vac feeding out to a NAS server, main PC, 8 port TP-Link Gigabit switch which hardwires to the Marantz NA6006/TV/Blu-Ray player, and a TP-link powerline adaptor that feeds the wireless over to the other side of the Apartment.
All I can say, is many thanks for pointing to the IGMP settings as due to tweaking those seems to have resolved all the issues I was experiencing.
Tony.