I noticed that my old backup files were not being deleted despite my Cleanup Task being set to delete all bak files more than a day old. Having checked, and double checked the path, file age, etc. I decided it was time to try a couple of random things.
Firstly I added a \ to the file path, this seemed to make no difference. So I then decided to remove the . before the bak extension (put there by SQL Server).
All is now working as expected.