Hi Karl,
Looked in 1861 Colchester index, but no joy

. Also searched all Essex resources I have for 1841 without success, BUT I don't have an index for Colchester 1841.

Did you see on IGI:
THOMAS THORN
Male
Christening: 13 SEP 1829 Saint Mary The Virgin At The Walls, Colchester, Essex, England
Parents:
Father: THOMAS THORN Family
Mother: CATHERINE
Source Information:
Batch No.: Dates: Source Call No.: Type: Printout Call No.: Type:
J120111 1561 - 1885 NONE Book NONE
Sheet: 00
Here are the Thorn families that I found in Colchester in 1861 (RG9) as they may be of some use?
Surname Forenames Age Piece Folio Parish
THORN ALFRED 28 1097 137 CCB
THORN HARRIET 50 1098 99 CCT
THORN HENRY 56 1098 99 CCT
THORN JOHN 48 1098 20 CCG
THORN JOHN F 17 1098 20 CCG
THORN MARY 59 1099 107 CCP
THORN MARY ANN 40 1098 70 CCG
THORN RICHARD 51 1098 70 CCG
THORN SARAH 54 1098 20 CCG
THORN SARAH A 15 1098 20 CCG
THORN WILLIAM J 10 1098 20 CCG
Parish Name Code Piece Folios Population
Holy Trinity CCT 1098 83-104 875
Saint Botolph CCB 1097 4-166 5,995
Saint Giles CCG 1098 10-78 2,714
Saint Peter CCP 1099 72-126 2,130
Total Population (16 Parishes) 23,482
And from FreeBMD for yesterdays(?):
Births Mar 1871 Thorn Henry Thomas Lewisham 1d 894
Births Sep 1873 Thorn Thomas Arthur Lewisham 1d 870
Good hunting,

Geoff.