According to the Pizza Express page the waiter should be getting 70% with 30% going to the cleaners and kitchen staff.
If tax is deducted I suppose that would make it about 50%.
All cash tips go straight to your waiter. But credit card tips and our service charge tips are different. Tips made on electronic card payments are fully distributed among employees as agreed by the company's Tronc committee, with no deductions. Currently, this independent group elects to give 30 per cent of card-based tips to cleaners, pizzaiolos and other kitchen staff; with the remaining 70 per cent going to the individual waiter as a reward for great customer service.
https://www.pizzaexpress.com/help-and-contact/where-do-my-tips-go 

" For parties of 7 or more adults (18+) we will automatically add a discretionary service charge of 12.5% to the bill.  Also, please note that our Jazz and Event sites will add a discretionary service charge of 12.5% to every bill.  

This charge is completely optional, so please ask your waiter if you would like this to be removed."
https://www.pizzaexpress.com/help-and-contact/do-you-add-a-service-charge-to-the-bill