I had read some speed checkers can not cope with the high speeds of full fibre very well.
I have tested over, Fast.com, Think Broadband, Ookla and they ranged from 550mpbs to 492mpbs .
The new BT Wholesale test is consistently lower at around 329mpbs
Do you any ideas from experience which site will be nearer the correct figure or if none of the above, can you suggest
one to use for a reliable reading.
If you have steam, download something from your library.
Failing that, task managers performance tab will give you the up and down speeds that your network card is seeing.