Paying HMRC Self Assessment Tax - by cheque, with or without a pay slip, over the telephone or electronically online payment;
K identifies tax paid as self assessment personal tax.

Late filing penalties are separate from late payment penalties

about Late filing penalties

Methods of making personal tax payment to hmrc:

Self Assessment payment of personal tax the traditional way

Paying tax by cheque to HMRC with the January and July tax payment slip
Send a cheque with the preprinted payslip in the preprinted envelope. Write your ten figure UTR Unique Taxpayer Reference, as payment reference, adding a K at the end, on the back of the HMRC cheque. Preprinted pay slips my also be used for paying across bank counters, allow three working days.
You can no longer pay over Post Office counters. This facility was discontinued 15th December 2017.
HMRC Banking details shown on preprinted pay slips should not be used for electronic online payments. See details below.

If you have no personal tax pay slip from HMRC
If you wish to pay by post but do not have a preprinted payslip, write a note explaining:
That you are paying Tax under Self Assessment
Quoting your personal tax ten figure UTR Unique Taxpayer Reference , adding a K at the end.
Cheque HMRC: Write your UTR Unique Taxpayer Reference on the back of the cheque, adding a K at the end,
Where to send your tax cheque:
Send this to HMRC Non-geographic post code:
HMRC
Direct
BX5 5BD

What is a Non-geographic post code? Check with Wikipedia

Print out a payment slip:
There is also the HMRC Self Assessment payment slip to print out and send by post.

Preprinted personal tax paperwork is mailed to your home address by HM Revenue & Customs.
If your address has changed this will not be picked up by HM Revenue & Customs unless you, or your accountant, have notified your Self Assessment Tax office.
Perpetual reorganization of Tax offices may mean that changes have not been implemented even if you have told them.
A change of address may not be picked up from your Self Assessment Tax return.

Paying Tax due under Self Assessment on your debit card by telephone - Service discontinued

for debit cards electronically online and credit cards see below

Self Assessment Tax personal tax

You should always quote your ten figure personal tax UTR Unique Taxpayer Reference without gaps and adding a K at the end.

Payments need to be directed to either HMRC Cumbernauld or HMRC Shipley except for Worldpay see below one of which will be your allocated pay office. Neither of these is your personal tax office for Self Assessment Tax purposes. The HMRC Shipley or Cumbernauld problem is also applicable to Corporation Tax and PAYE/CIS

To establish whether your pay office is either Cumbernauld or Shipley you need to check your paperwork. If this is not possible HMRC ask for payments to be made to Cumbernauld. There is no penalty for paying the wrong office and you can expect your funds to be correctly allocated if you provide the right reference number.
If you are able to contact your Self Assessment Tax office they should be able to tell you but low service capacity can make telephone contact extremely difficult.

HMRC's online electronic bank account details for Self Assessment Personal tax
- if you are a self employed sole trader the same as for PAYE NIC and CIS

Cumbernauld Accounts Office references
HMRC bank: Sort code: 08 32 10
Account number: 12001039
Account name: HMRC Cumbernauld
Payments from abroad quote:
International Bank Account Number (IBAN) GB74CITI08321012001039
SWIFT Bank Identifier Code (BIC): CITIGB2L

Shipley Accounts Office references
HMRC bank: Sort code: 08 32 10
Account number: 12001020
Account name: HMRC Shipley
Payments from abroad quote:
International Bank Account Number (IBAN) GB05CITI08321012001020
SWIFT Bank Identifier Code (BIC): CITIGB2L

Paying Self Assessment Tax by internet, telephone banking or BACS Direct Credit

Normally takes three bank working days for payment to reach HMRC

Worldpay:
Paying Self Assessment Tax by debit card or credit card over the internet
(American Express Amex or Diners Club are not accepted)
No credit cards are accepted after 13th January 2018

The payment accounts office Cumbernauld or Shipley is not specified using this method

Pay HMRC by debit card - no charge.
Credit card payments are subject to a non-refundable transaction fee according to the card used.

Worldpay it is counted as paid the same day; although the bank processing time could be three to five days.
The HMRC Help Line telephone number has been discontinued.

To go into HMRC Worldpay: https://www.tax.service.gov.uk/pay-online/self-assessment
Just enter your UTR unique taxpayer reference:
Ten figures
With “K” at the end for self assessment
The amount £
Then go through the card procedure like any other online payment.

Paying Self Assessment Tax by CHAPS transfer

Pay tax by bank transfer
Same day provided you initiate payment within the time specified by your bank, usually between 9.00 am and 3.00 pm.

By Direct Debit

If your cash-flow is sufficiently robust you can pay by direct debit. Find out how at https://www.gov.uk/pay-self-assessment-tax-bill/direct-debit

Budget payment plan - You can also use this method to make regular payments in advance of the tax liability falling due.

The HMRC Help Line telephone number has been discontinued:
For if you need both time to pay an existing liability and wish to progress onto making regular payments in advance.
Access the advance payment service throught the Direct Debit set-up procedure when you are up to date with previous payments.

On account payments for Tax year 2018/19

Normally payable when untaxed income exceeds £1,000 per year
Payments on account Payable:
31st January 2019.
31st July 2019.
No penalties for late payment but daily interest is charged.
Mathmatical basis of calculation; two amounts of 50% of 2017/18 repeating liability.
Claim to reduce these amounts if you think the figure is too high.
But arrears of interest will be charged if you go below what turns out to be the actual liability.
File the 2018/19 tax return before 31st July 2019 to be sure you are paying the right amount of tax.

Paying HMRC Self Assessment Tax - by cheque, with or without a pay slip, over the telephone or electronically online payment; K identifies tax paid as self assessment personal tax.