@Iggy888 

For the purpose of speed testing, force Deco Main network to run on 5GHz only:

After that, run speed tests again.

In my speed tests for M5 running in AP mode where everything related to bandwidth throttling and content filtering is disabled, with 5GHz band only, I am getting up to 500Mbps WiFi speed on gigabit Internet link. More than enough for your Internet speeds.

You will need ISP router to run Deco mesh in Access Point mode. If you can't figure why WiFi speeds are slow and nothing helps, wait till you get BT's new router, connect Main Deco to BT router, configure Deco mesh to run in AP mode and run speed tests again. Switching a Deco to Access Point (AP) Mode

Thanks for your help @Alexandre.,
I switched to 5Ghz only and ran again, using Speedchecker app, it shows 423.99Mbps to router but only 47.89 Mbps Download over wifi on my mobile.
On a laptop Broadband Speedchecker is showing 58.78 download (but a confusingly high upload speed of 851.82)  

Will try AP when I get the new router from BT. 

Iggy888 wrote

Thanks for your help @Alexandre.,
I switched to 5Ghz only and ran again, using Speedchecker app, it shows 423.99Mbps to router but only 47.89 Mbps Download over wifi on my mobile.
On a laptop Broadband Speedchecker is showing 58.78 download (but a confusingly high upload speed of 851.82)  

 

  @Iggy888 

Wait, how can you get (some of) speeds in excess of your Internet link speed, which is if I googled "BT Full Fibre 100" correctly is 100Mbps???

  @Iggy888 

For speed testing, install Ookla speed test app, which is available in Windows app store, Apple app store and Google app store.

  @Iggy888 

I would trust Ookla speed test app in that case. Results are still not good. 

In Deco Router mode, a lot could impact speeds. If you have preferred/priority device, remove that. If you have QoS, increase it. Virus and content filtering might impact speeds, as well.

You can go through Deco Router settings and try to reset (undo) those that apply to content filtering, bandwidth throttling, device pioritization. Alternatively, wait till you have BT router and by switching Deco mesh in AP mode as I suggested you'll get rid of all these impairments and will be testing raw WiFi speeds Deco M5 can deliver.

  @Alexandre. Yeah I'd already taken QOS to 1000Mbps and removed any prioritisation. The only things connected to wifi at the moment is my mobile and a few smart plugs (and my Sky box, but turned off). Without a router anything connected to a switch is out. Virus filtering was off too.
Before FTTP, everything was great, pretty much 72-75 Mbps everywhere.
Will try again when the new Smart Hub arrives, failing that, may invest in a Vigor 2765.
I appreciate your help.

  @Alexandre. 
OK new Smart Hub from BT. After a bit of faffing, got the Mesh working.....getting 140-150Mbps - yey!!... or so I thought.
Living Room (main Deco) 140-150, Main Bedroom Deco (140-150), Office Deco (adjacent bedroom - 50-60). If I force Office devices (two laptops and a printer) to Main Bedroom from the Office I still get 50-60.
Could my third Deco have an issue? 
Smart Hub wifi is turned off.

  @Iggy888 

The good news is your Main Deco delivers expected speeds. To find what is going with your Office Deco, please share "Show List" and "Show Map" screenshots from your Deco app.

Examples:

and

Also, for Office Deco open its info page in Deco app, make its screenshot and share, it too. Example:

Thanks  @Alexandre.