UK Landlord Forum / Life

couple who are renting are 2 months in arrears with rent.  Have had trouble contacting them, then different excuses for non payment. Started saying that property was damp.  I had it checked out and was told it was condensation.  They then had environmental health visit who confirmed what builder had said (that it was condensation). The male tenant then said that he had no idea of the rent arrears that it was his partner who should have been paying but that he would clear arrears on 24th Jan when he got paid. I decided to give them a chance and wait to see if arrears were paid and put this in a letter which I sent by recorded delivery. 
Today (22nd Jan) received a letter from them giving notice that they would vacate the property on 21st Feb and that they knew that they had not cleared the rent arrears yet but would pay this in full and any rent owed by 21st Feb.
I am not stupid and realise that this is just a delaying tactic and that by the 21st Feb they will probably already have vacated property and then I would be left with 3 months arrears of rent owed and no forwarding address
I have contacted a solicitor who has advised me to send them letter saying that I do not accept this and give them 7 days to pay these arrears as promised or I would start legal proceedings
My predicament is that I am unable to connect laptop to a printer at moment, will it be ok for me to compose letter, email to my sister who lives near property and let her pop through their letterbox.  It will obviously not have my signature on it but would it be ok for my sister to sign on behalf of me. I am aware that if proceedings go to court this letter may be used as some kind of evidence.  Time is crucial at moment and I would prefer them to get reply tomorrow rather than next Mon or Tues

yes, she can "pp" aka "per pro" the signature.