Help with only 4.2MBS on Superpowered Fibre

Hello everyone this is my first visit here.

I have SimplyBroadband with Superpowered Fibre since October. At first I was only getting 450kbs download. After arranging 2 engineer visits I am still only getting a maximum of 4.2Mbs. I got a text after the engineer visit asking if fixed and I replied NOT FIXED but nothing happened. Help please - is it worth trying anything further!

thanks

Graham