The following may work.
If you hold down the shift key then tick the box to the left of the first email you want to delete in the list of emails then scroll down using the scroll bar on the right hand side or use the PgDown key and tick the last email you want to delete then release the shift key.
It will select all the emails in between.
You can then click delete or drag and drop them into the Trash folder.
Thanks for your advice and I will try this.
Ive tried this now and unfortunately it does not work.
In case you were trying to delete too many at the one time try it with batches of less than 200.
The other way to do it is to install an email client such as Thunderbird and set it up as an IMAP account and it will allow you to delete more than 50 at a time. It synchs with the BT servers so will delete them in webmail.
Thank you again. I will give it a go.