DX number

[SodaPop]

I am sending a form to companies house (can't file online at the moment) and at the bottom of the standard address there is a DX number.

I am not subscribed to the DX service, so do I put it on the envelope too (not sure if the receiver and sender need to be subscribers).

Would appreciate any help with this.

Thanks

EDIT: Never mind, I just called DX to ask, and they confirmed that unless I am a subscriber I just ignore the number.