I have had to swap the locations of my Plusnet router and TP extender. The router is now close to line into the house downstairs and the TP extender is just at the top of the stairs. According to the indicator lights there is a good signal to the extender. My devices show good signal strength all around the house now - better than before. Everything is fine whilst I am in range of the router downstairs but, in spite of the signal strength, the WiFi is frequently drops when upstairs.
According to the admin of my router, all my devices are connected to the 2.4Ghz and nothing to the 5Ghz.
I am afraid I do not understand the finer points (5Ghz vs. 2.4Ghz; channels etc) in some of the posts here on the forum.
Could it be that the extender is faulty? The connection to the TP extender was iffy when it was downstairs and the router was upstairs, but it did not drop out nearly as much as now.