I am using KDE and have foolishly deleted a vital directory and then even more foolishly emptied the Wastebin, can I recover files from an emptied wastebin?
I assume it is better to restore from your last backup.
Yes, normally I would but the last couple of days have been a rush to modify software for a customer away from the office so no backup 
I can recover from the backup and modify the files as I have rough notes but obviously can’t be sure that they are exactly the same as the customer has which is an issue. Hence, the question about recovering the emptied wastebin on my system.
To get the highest chance to recover these files you would need to stop using this hdd/ssd immediately.That means stop writing ANY data to this hdd/ssd. (The best solution would be to unmount the drive and put it into an other pc as read only). After that you can use programs like PhotoRec and try your luck.