His birth index:

First name(s)         Nicholas S
Last name             Tanner
Birth quarter         1
Birth year            1955
Mother's maiden name  Shelley
District              Oswestry
County                Shropshire
Country               England
Volume                9A
Page                  169
Record set            England & Wales Births 1837-2006
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There are four marriages for Nicholas S Tanner:

Last name  First name(s)  Year  Location
Tanner     Nicholas S     1980  Colchester, Essex
Tanner     Nicholas S     1989  Ipswich, Suffolk
Tanner     Nicholas S     1993  Andover, Hampshire
Tanner     Nicholas S     2002  Ystradgynlais, Breconshire, Wales

The fairly obvious one is the first because Colchester is where his father was living at his death in 1994.  But a little bit better evidence is needed. and I think it has to be from the marriage certificate, which I shall order.
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BP reports “Nicholas Shelley [Nicholas Tanner Esq, The Coach House, Ampney Crucis,
            Cirencester, Glos GL7 5RY]; b 7 Jan 1955; educ Harrow;
            m 21 June 1980 ●Melanie Anne, er dau of A F Challis, and has had:
              George William; b 4 July 1982; d 26 May 1989
              William Archie Bowen; b 24 June 1988
              Olivia; b 16 March 1990
              Isabel; b 10 June 1993”
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FindMyPast lists exactly those four children from a search for all Tanner births with a mother’s maiden name of Challis between 1980 and 2000, though it does add to the forenames:

Last name  First name(s)       Year Of Birth  Location
Tanner     George William      1982           Swindon, Wiltshire
Tanner     William Archie B    1988           Cheltenham, Gloucestershire
Tanner     Olivia Cicely R     1990           Cheltenham, Gloucestershire
Tanner     Isabel Charlotte A  1993           Cheltenham, Gloucestershire

And the online GRO records are, for the last three only currently:

Name:                        Mother's Maiden Surname:
TANNER, WILLIAM ARCHIE B     CHALLIS  
GRO Reference:  DOR  Q3/1988 in CHELTENHAM  (4791A)  Volume 22  Page 1983

Name:                       Mother's Maiden Surname:
TANNER, OLIVIA CICELY R     CHALLIS  
GRO Reference:  DOR  Q2/1990 in CHELTENHAM  (4791B)  Volume 22  Page 2105  Entry Number 18

Name:                             Mother's Maiden Surname:
TANNER, ISABEL CHARLOTTE ALICE    CHALLIS  
GRO Reference:  DOR  Q2/1993 in CHELTENHAM  (4791B)  Page 0  Reg B45D  Entry Number 275
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Their marriage index:

First name(s)       Nicholas S
Last name           Tanner
Marriage quarter    2
Marriage year       1980
Marriage Finder     Nicholas S Tanner married one of these people
                    Melanie A Challis
Spouse's last name  Challis
District            Colchester
County              Essex
Country             England
Volume              9
Page number         2336
Record set          England & Wales Marriages 1837-2005
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