Contact the Probate Registry and order as many copies of the 'grant of representation' as you think that you might need. The 'grant' will be 'probate' if your fiancé left a will, or 'letters of administration' if not.
The first copy costs £5 but additional copies are just £1 each. Then send one copy to each of the organisations which are writing to you, using recorded delivery. Enclose a letter stating;
"Dear Sir
I write with regard to the estate of the late Mr Xxxxxx Yyyyy, formerly of this address. Please note that the grant of representation is held by Mrs Zzzzz Yyyyy of 23 Acacia Avenue, Sometown, Anyshire, SN1 1AB. I enclose a copy of the grant for your information.
Please send all further correspondence in this matter directly to Mrs Zzzzz Yyyyyy, or to her legal representatives, Mssrs Bodgit & Meddler of 17, High Street, Sometown, Anyshire, SN3 4XY".
http://www.hmcourts-s...e.gov.uk/cms/1226.htm
Chris