1. After the engineer went do you still have a noisy line?
2. Are you using a corded phone to do a quiet line test?
3. Is every device except the corded phone connected when you do the test?
If the answer to 1. is yes, and you've done 2. and 3. Then there still is a fault somewhere.
If the answer to 1. is no, and you've done 2. and 3. Then that is the maximum your line is capable of.
I suffered from the same problem. When I first got fttc I was in the mid to low 70s. Now I'm at 53 and I'm going to be dropping the higher tier package I pay for. Sadly this is a reality of fttc. It's all based on electricity and there is a lot of electrical interference in the fttc cabinets, so the more people sign up for fttc the more interference everyone gets and the slower speeds we get. If your using an isp that goes through the openreach fttc cabinet then the same issue will be there. Like I'm on BT and lost 10Mbps when a new neighbour moved in and signed up to Sky. Both lines are using the same fttc cabinet and then there is the usual interference of the copper lines from the cabinet to the house (fibre to the cabinet means its fibre from the phone exchange to that cabinet, everything else is on copper lines).
So if you have no noise and still getting slow speeds then try a different technology, mobile packages, virgin media, or if you're lucky enough to be able to get full fibre (fttp or ftth) then get that.