BT WiFi is disabled now, and it sadly did not work as expected.
Tried the same test and it didn't solve the ping problem. It's weird, the fact the upload from the public WiFi has minimal impact with devices on the private network, surely that proves that the Smart Hub is capable of ensuring other devices are not impacted negatively - so why can't the Smart Hub do that on the private network, more so even with BT WiFi disabled?
My understanding is none of the recent homehubs have had Quality Of Service (QOS) for the private network, all they have is QOS for the Bt WiFi so that it only uses spare bandwidth and doesn't impact on the private network.
I suspect there is no QOS available as we are not expected to be able to configure it sensibly and therefore lead to extra support calls.
Being of 3Mb ADSLmax, we frequenty staturate the available bandwidth, so while leaving the HH4 to connect to BT, the internal network connects to it via a TPLink TL-R470T+, and all traffic uses the QOS facilities on it to share out the bandwidth in a bit more equatable manner.