I have been using Clockbase for a few months now, after using the free trial i signed up and am already seeing an increase in the amount of billable hours i am now invoicing.
Simple to use, basically at the start of each day get your employees to login and begin work on a certain project, a timer runs and logs how many hours they spend each day on specific projects (you can add multiple).
They can also add any expenses incurred relating to that project. Then at the end of each day/week/period you can simply click a button and turn all these hours and expenses into billable ready to go invoices.
It has saved me hours messing around, and even more in the amount of hours i had not billed for.
Highly recommend going to https://www.clockbase.co.uk/ and giving it a try.