Bridge has suddenly started to break network

[AndyOB]

Hi,
bought and installed a Bridge and 2 S5s about 8 months ago. Plugged them in and off we go - no problems.
Now this morning I suddenly found having severe network issues - no, or severely slow, disrupted internet - windows unable to create network maps - unable to connect to the network printer etc.
By a process of Sherlock Homes like deduction (turning stuff off/on) I have narrowed the problem down to the Sonos bridge. If this is off then everything works fine like yesterday - if on - back to nothing working.

I powered the S5s off - and the bridge on - one thing that's noticeable is that the green ethernet activity light goes on practically solidly - even though I'd have though nothing should be happening if no S5s asking for traffic?

Anyway - I'd appreciate any assistance in how I can determine the issue

Network setup
Sonos Bridge plugged into D-Link DIR-615 wifi router (DHCP off)
Which in turn is plugged into a D-link DGS 1008D switch
Which is in turn plugged into a D-Link DIR 655 Router (wireless disabled, DHCP on)

Point is nothing changed between yesterday - working - and today - not working in terms of network setup....