Contract for Fibre Broadband who is reponsible

Sounds like SIL fitted a range extender, not a powerline adaptor, a powerline adaptor would be almost full strength signal, but would need cable to laptop/PC not wifi.
They do work well, I have one in the living room, other end to the router, saves trailing a cable,plus one in the workshop about 150 feet away at the end of the garden, works with no weakening of signal. Wherever there is a 3 pin plug you can have a powerline adaptor.
A range extender on the other hand can sill be foiled and needs optimum conditions to be worthwhile.
Ideally a full system and cabling check and firmware updating is the right way to start.

Some weeks ago for whatever reason I could not get into my laptop password and had to get someone in the sort this out for me which he did.
He also solved my Wifi problem by setting up a Wifi extender in the diner which now provides Wifi to our bedroom, I am so pleased.

The provider's responsibility is to provide a service to the router - and that is all. Whether it can be received anywhere else in the house depends on the house and where the router is located.

As far as the dropping out goes. Do you have a fibre connection directly to the house? Or do you, like many people only have a fibre based broad band service as far as the nearest BT green cabinet with only a copper wire connection from there to the house?

We live in a very old house with a huge double chimney stack in the centre of the house acting like a wall between the two halves of the house. Study and router are one side of the chimney, LR the other. We have tried a few alternatives to getting internet access in the LR, but with only partial success.

We have a holiday house in France with 3 foot thick stone walls. Here we have been able to put the router in the roof space, above all the walls and that gives house-wide internet access.

Your problem is nothing to do with your supplier. They have fulfilled their part of the contract by supplying an internet service to your one place in your house

It sounds as if your SIL connected a WiFi Powerline adapter from your lounge to your diner. It runs a cable from your router to a nearby electric socket. The signal from your router actually travels along your mains electricity circuit and is picked up by the second adapter plugged into an electricity socket in your diner. The adapter in your diner then acts as a second router, sending and receiving WiFi signals (I assume that you are using WiFi in your diner, otherwise you will have a cable running from the plug in your diner wall socket to one of your computing devices in the diner). If you unplug the adapter in your diner and plug it into a wall socket in your bedroom, you will have, in effect, moved your router extension from your diner to your bedroom. If you try it, and it works, you will know that you need a WiFi extender and it is not down to TalkTalk.

Perhaps your SIL can help you again?

Unfortunately the router is in the lounge which is at the other end of the bungalow to the bedroom.
The phone line comes into the lounge and I have no idea how you could extend it, perhaps someone else knows.
I am not technical in any way so don't really know how these things work.

Is it possible to move your router? Ours was in the kitchen and the signal was very poor in the living room, but I moved it into the hall and it is better (a few messy wires visible that I would rather not have, but .. hey ho)

Have you tried moving the extender from the socket by the TV to the main bedroom? If that works, you will know that it is not TalkTalk's fault but a problem with either signal interference or obstruction in your house.

I am going to try and get some information on connecting another device to connect to the one in our diner.
I have had a long talk with another technician at Talk Talk and he was more helpful than the one the other day.
He talked me through changing the settings on the router, it improved at first but still droped out. But he said to keep trying so I will to see if it makes a difference.
It's just droped out again while I have been typing this ,its very frustrating.

You can download an app that will test your broadband speed. I don’t think any provider is responsible for the coverage in every room of the house only for the overall speed of your Wi-Fi. We have had to plug a booster in - one near the router and one downstairs by the tv. It would be different if the Wi-Fi was poor in every room.