Slow iOS / iPhone 6 Plus / iPhone 6 / iPad 2 wifi speeds using Netgear R7500 Router and Virgin Media Superhub2 as Modem)

I have the bizarre situation that I get the following speeds:

Virgin Media SuperHub 2 AC (set up to use as a Modem) - latest Firmware

Netgear Nighthawk X4 R7500 as Router - latest firmware

Sony Viao Laptop (windows 7): (Wired) - 160Mbps & (5 Ghz Wifi) - 140Mbps

Toshiba Laptop (wired) - 160 Mbps (windows 7)

iPhone 6 Plus (5Ghz wifi) - 35Mbps-50Mbps (though it often starts out at 100Mbps but then rapidly drops)

iPhone 6 (5Ghz wifi) - 35Mbps-50Mbps (though it often starts out at 100Mbps but then rapidly drops)

iPad 2 (5Ghz wifi) - 35Mbps-50Mbps (though it often starts out at 100Mbps but then rapidly drops)

Speed test through SPeedtest.net app!

I have reset the network settings on my iPad 2 - this had not effect (I haven't reset network settings on the phones as I have been told this may cause problems with my mobile network settings). I have reset the router and started from scratch.

I have checked all channels and there is little interference there. The router(s) has/have been moved away from speakers and other potential interference and when speed tested there was no other data drains on the routers.

In addition - when using Wifi SweetSpots app on myiphone 6 Plus when on the 5Ghz band it shows a possible throughput of 274.3Mbps.

SO there must be something in the router settings or the iOS settings that is stopping actual data from reaching my iPhones or iPads with proper speeds.

So far I have tried each of the following with no difference to the outcome:

- Check security is WPA2

- Turned off airplay

- Turned off QoS

- Turned off Greenfield

- Changed channels

- Changed channel speeds under each 2.4 and 5GHz bands

Any other ideas? Please help

MM13, London UK