It's second instalment of self assessment tax time and HMRC are no longer issuing reminders. Cue lots of "how much was it again?" calls. I think clients know but are hoping the amount has somehow reduced!
Having said that, if you have completed your 2018 Tax Return, then your actual liability can be calculated and if less, then a claim to reduce can be made. If the liability is in fact higher than the payment on account, the difference is not due for payment unitl January 2019, so no reason not to submit early.