Tax due - no UTR

Pensioner client's wife has CGT due, not on SA as only small pension income.

The couple sold there holiday home and there is a capital gain arising.

I submitted an SA1 and have prepared the Tax Return and computation.

Client has given me a cheque payable to HMRC for the CGT.

So, I think as far as the return is concerned in due course HMRC will ask for a return and I will have three months to file.

What about the tax, should I send the cheque with a covering letter and her NI number on the back, or do I wait for a UTR?