The eventual solution was that a BT engineer turned up because the flat upstairs was having outages and his first instinct was to change the wires from the house to the telephone pole. His instinct was correct and so it took him about 45 mins to correct a problem that had blighted me for years.
That was about a year ago and the broadband has had no rainfall associated dropouts since.
That was about a year ago and the broadband has had no rainfall associated dropouts since.