I am running a Mesh System based on 5 Vigor 903's 4 have wired uplinks and one is WiFi. My question is regarding what channels to use for my WiFi.
I read somewhere that on a mesh system all channels for the root and the nodes should be the same. i.e. 4.2 GHZ all the same and 5GHZ all the same, which I would have thought may not be the right option. Or should I set the channels on each device to the least crowded channel? I am aware there may be many factors but if there is a simple answer I would appreciate it.

In my view because the Draytek "mesh" system is a sort of half-hearted affair, being based on WDS and not using a dedicated separate wireless connection for the uplink, the uplink/downlink channels HAVE to be the same on all wireless connected devices - see here for example https://www.draytek.com/support/knowledge-base/7509

I use the same channels on both 5GHz and 2.4GHz (2927ax root and 3 AP906 nodes) and this doesn't appear to cause any issues. Whether or not the performance is any different to a "traditional" WDS set up is moot.

I must admit I initially tried putting all the 903's to difference channels and it seemed to work and I didn't notice any difference when I changed them all to use the same channels for each frequency.
I see you are using your router 2927ax as the root. My router is the 2027ac and I initially tried to set the mesh up using that as the root. But I started experiencing some weird network issues where the 2927ac seemed to freeze for a while. So I decided in the end to leave it out of the mesh setup.
My 2927ac is connected to a Virgin Hub 3
Looking at that article in your response that seemed to imply that all nodes needed to be on channel 40 because of the backhaul. Mine was on 36 which it chose by default. But then again it worked.
4 of the 903 I have are wired uplink and just one is WiFi only.