PlusNet are saying it is bufferbloat (which I had never heard of).
Seems strange to me that it has only manifested itself in the last 2 days...
Are you able to comment please ?
Peter

Buffer bloat explains the long latency during an upload.

But you said that upload was failing completely.

The two are contradictory. If you are seeing long latency during an upload, it means that the upload is proceeding.

Thanks for your response candlerb.
Sorry if I inferred that the upload was failing. By 'it is not happening' I mean it doesn't progress very much. For instance I tried to upload the same 1Gb file to Dropbox. It started off at about 1 minute uploading time but, after a few seconds, it changed to 9 days (yes 9 days).
Not sure what I can do about the bloat - especially as it has suddenly appeared.

Goto https://www.thinkbroadband.com/speedtest and postcode the results page from the results.

Buffer bloat may be support grabbing at something from crib sheet.

1GB on 5 Mbps would take around 26 minutes to upload.

It might just be a problem with dropbox, hence the speed test request which also measures bufferbloat

Here is the result from speed test:
https://www.thinkbroadband.com/speedtest/15742700692...

Looks okay to my untrained eye.

Wondering if 'support grabbing at something from crib sheet' may be true.
Have tried uploading the file on a neighbours BT connection and works perfectly.
Will borrow his BT hub and try here. If the same result then it 'must' be PlusNet.
If not then could well be my modem/router.
Also please note the problem first came to light uploading to Google Drive. I tried the
upload on dropbox to see if the problem was still there, which it is.

Hanging uploads does sound like an MTU issue.

As experiment: on your PC try reducing the MTU of the ethernet card to 1400.

If the problem goes away, then there are better ways of solving it which involve your router configuration.

Thanks but changing to mtu1400 from 1500 made no difference

The bits around buffer bloat you may have missed

when idle average latency for small TCP packets was 55ms
When downloading this rises to 222ms
When uploading this rises to 75ms

Nothing horrendous there, yes there is buffer bloat but to be expected unless you mitigate it, which routers don't do out of the box generally.

Only comment would be - run the test a couple more times as the upload speed did vary, so its possible something did try to upload some data at that time.

how did you make the change and which OS ?

So the question again is what are the sync speeds reported by the modem?

The modem says;
Downstream: 46.9 Mbps
Upstream: 16.27 Mbps

Tbb speedtest still hangs at stage 3.
Speedtest.net now completes but reports upload speed of 0.05 Mbps

So I bit the bullet and contacted CS at Plusnet. They ran a line test and found there is high resistance on the cable to my house. Awaiting Openreach tech tomorrow morning.