Prior to a week and half ago was the WiFI working fine and was you using the same ISP and same router or have you changed provider and router recently or moved the router into a different location in your home ?
Is NowTV your broadband provider and if you have already done a 12 month contract with them, have you re signed up again with NowTV for a further 12 months on their super saver broadband deal ?
If you are with NowTV are you using the Hub 1 or Hub 2 router where the Hub 2 router is significantly better over wifi ?
Did the Engineer test & confirm that the incoming internet speeds are what they expected ?
What download and upload speeds was you given as an estimate when you signed up and what speeds are you measuring at the router ?
Have you tried an Ethernet cable between the router and any of your devices to see if there is any big improvement over WiFI ?
If your router is dual band, i am guessing you have tried different frequencies bands and channels in the router settings.
What's the wifi signal like when you are using a device in the same room as your router ?
So NowTV are your broadband provider ?
If you have been with NowTV for over a year then I am assuming you are using the Hub 1 router ?
I am guessing you have tried pressing the reset button on the Hub Router to see if there is any improvement over WiFI ?
Suppose there is always a small chance that the Hub Router has developed a fault with it's inbuilt wifi antenna, where the BT Open Reach Engineer will not check the Router wifi because their responsibility is only up to the in coming master telephone socket.
Personally if it was me, i would contact the nowtv broadband team and ask them for further technical assistance.
There should be a telephone helpline number under My Account > My Package (moving home) and ask them to transfer you to the relevant broadband team.
Whilst on the phone to them, ask them to confirm if you are still on a fixed 12 month broadband contract or you are now on a non contract flexible agreement.
I am guessing the nowtv broadband technical dept have asked you to press the reset button on the back of the router ?
When you phone them back, raise the question "could it be possible that the Hub Router has developed a fault over wifi" and would they consider replacing it with a new Hub router (just to eliminate the router out of the equation that is causing you the wifi problems at your home).
I would still get confirmation from nowtv that you are on a fixed broadband contract or not.