76Mb fibre only has downstream under 50k

Hi

I've used to have 38Mb fibre and could get speeds of 36-37Mb.

I've now upgraded 76Mb fibre but now I'm only getting a speed test result of 46Mb on the PC and I expected it to be higher.  The Downstream line rate on the router is 49,582.

I've gone through all the speed check help, e.g. powering down the router right through to removing the master socket but still only get a downstream speed of around 50,000.

Should I expect higher or is this the highest my exchange could give me?  I was advised the uprade would get me up to 70.7Mb

Thanks