Firmware updates are totally under the control of BT and can't be reverted to a previous version by the user. Hence many of us use third party routers.
Depends on your budget and requirements really. If you on want a cheap 'n cheerful router with only 2.4Ghz wireless, the TP-Link TD-W9970 is only about £30. If you want something a bit better with 5Ghz wireless have a look at the TP Link VR600 or VR900 or the ASUS DSL AC56U or AC68U. Whatever you get, make sure it is a VDSL modem/router.
Even if you recontracted and BT sent out a new hub you wouldn't get it before Wednesday.
We have tried a factory reset, and also just switched everything off overnight.
This morning it is still the same.
Changed filters as suggested by helpdesk.