@David-TP 

Thanks for the response!

The router was provided by the ISP approx a year ago. All has been working fine up until about 2 days ago, with no changes having been made.

How would I go about testing the Ethernet Speed from the actual router? I have done the diagnostic tests in the Models advanced settings but says all is okay. I have tried mulitple ethernet cables as well incase one is damaged. 

I have another modem/router being delivered today to see if i swap it out with the same settings I get better/Normal speeds, then I can at least go back to the ISP and require a replacement. Right now they won't do anything without hefty charge as their tests state the connection to the router is fine. 

Quick update for anyone who may be following or in the future has similar issue and comes across this via search engine.

This evening I tried a brand new router, new power adapter, new dsl cable, new Ethernet cable and got the exact same. Super slow download speed and normal upload.

I have raised this with my ISP who have in turn raised a ticket with OpenReach. Hopefully they have more in-depth testing to see why even though, two separate routers are registering a high downstream value from the cabinet, that they can only provide such a slow download speed. 
 

Will update again after engineer has been. Whenever that will be.