I have around 500 clients and only find that a handful actually get the old SA302 (tax calculation).
As it is now (and has been for several years) called "Self Assessment" HMRC place the onus on you or your accountant to calculate the tax due. It is then down to you to pay this when is required.
...not sure why those handful of clients still get the SA302. Must say I did prefer it when all clients got those!