@pscraven , I got lucky today. The Openreach engineer that came an hour ago seemed very knowledgeable. He had an app on his phone that showed each drop in the connection and he could see them happening every day.
As soon as I told him that the problem started when I changed provider he said he knew that might be causing it - faulty ports. He then went to the green cabinet on the street (and I went with him), disconnected my cable, requested new ports (via his app) which then appeared on a different card (that's the term he used). He then connected my cable to a new socket in the cabinet and said that I should be good to go.
He said the change was as if I'm a new customer, so there will be this stabilisation period of a week or so during which there could be occasional drops, but not at lunch time.
I'll monitor the situation in the next few days and report back here.
