We had tonnes of problems when we moved from broadband to fibre and it all ended up being related to a faulty telephone connection in our bedroom. It took 6 engineer visits to find that out and fix it, which they did to that their cost which I was not expecting as it was a fault in our house.

Your delivery method hasn't physically changed, just the location where it's overlaid onto your existing phone connection.
The reason why you've had problems is because the quicker your connection is, the less robust it becomes. Quicker speeds have to be delivered over correctly installed cabling, otherwise the end user doesn't get proper service delivery,

I do a lot of cable rearranging in my line of work, making sure that DSL services hit the correct point of delivery (ie where the router is going to be sited), and ensuring that subsequent is appropriately connected.

You'll need to find out where that first leg terminates.
If it is in the annex it will need crimping through to the main house, bypassing whatever's in the annex.
The more hops there are along the way the higher the overall impedance rises, which is not conducive to high speeds.

Infinity works best with the appropriate LJU  on the end, but there are dongles you can get.
They'll have VDSL on them.

The more recent master sockets and filters can be found here www.ebay.co.uk/itm/Genuine-OpenReach-BT-Master-Socket-NTE5C-VDSL-MK4-New-2016-Faceplate-BNIB-/332169094735?epid=569758785&hash=item4d56d2264f:g:~zIAAOSwx2VZhINX
They do not need any special tools to terminate, just a pair of cutters, and a screwdriver, and the connections are push with thumb crimps.

Make sure that the router position is the first hit in line, and any telephone extensions are connected to the appropriate point in the socket. It also eliminates the need for any more filters.

Well all I can say is I have had no problems, installation was a breeze but there was a coax cable here already, so it just needed a switch flicking. My Dad never had a problem (until he switched to BT for the Sport - what a BT nightmare that was) and my friend I have lived with on and off for nearly 2 years over the last decade has never had an issue with Virgin. Which magazine rates them higher than BT, but not by much and they are both in the middle of the pack rating wise for customer satisfaction. Personally over the last 10 years I have had a fair few issues with BT at 3 different addresses, they were all eventually solved but not quickly.

Despite the claims made in their relentless current marketing campaign (that bloody budgie ad is on at the movies every damn film), BT cannot compete with Virgin on speed, which was why I chose them. My package is 'up to' 200 MB/s and I get around 190 most of the time. It's not cheap though, I'm on a rolling monthly contract so can cancel at any-time, but it's £37.25 for that convenience.

Useful to know, Chris, I was informed that the faster performance resulted in it becoming less robust. Interestingly my stability has never been so good since they resolved the internal fault, it must have been an ongoing issue previously.

BT cocked up again. They got my phone number mixed up with someone else. I now have an engineer coming next week. 'Infinity' is still running at 7 Meg.

I suspect that we have multiple issues, the telephone wiring in the house and the Openreach run from the cabinet. Patience will win in the end.

I had a good one at the last house, he moved my master socket free of charge.

I hope you've made Openreach/BT aware that you won't be paying for the service until they have actually provided a service that meets their advertised parameters.