If they've had no contact for a few years they may be worried about the debt becoming statute barred (no contact for 6 years)
If you contact them they have another 6 years. Check the envelope it may not have been sent royal mail, if it has an S in a box then it is a company called UK Mail and their returned letters policy is,to scan the front of letters to show the company the reason for return and then destroy them to save costs for the company.
A default notice must be served,if its returned royal mail,it is un served as royal mail keep a record of returned mail
If its any other mail company then they will destroy the evidence and the company say its been sent and served.
So, write the usual " return to sender,addressee not recognised "
And " CEASE and DESIST "
And " Postage paid and certified "
And " The date " on the right hand side of envelope
Put a sticker over the window with their return address on it and who ever sent it (a personal name is preferable)
Write "opened by mistake on the back "
Put a first class stamp on it and take it into the post office and ask for a certificate of postage ( free )
Because it is sent to a P.O.box it is served
They will not bother you again and the default does'nt count
P.S if you want to put the CEO's name on then do "As of companies house"