Hello Nick,

Were Sydney Edward and Edith Mona Dawson married, or brother and sister? If married, what was her maiden name?

You said you have the death record about 1986 from Plymouth for Sydney E. His date of birth will be given in the cert and on the index entry. - Just checked the index and there are two Sydney (+ Sidney) Dawsons died 1984 and 1986 in Devon, Plymouth.
Name: DAWSON, Sidney Edward
Registration district: Plymouth
County: Devon
Year of registration: 1986
Month of registration: February
Date of Birth:21 August 1906
Volume no: 21
Page no: 2028
Reg no: 286

Name: DAWSON, Sydney Edward
Registration district: Plymouth
County: Devon
Year of registration: 1984
Month of registration: June
Date of Birth:15 May 1910
Volume no: 21
Page no: 1731
Reg no: 684

These are from Findmypast, but they will also be on FreeBMD (free).

I had a look at the 1911 census, which had:
DAUSON, Sidney Edward Son M 6 1905 School [born] East Finchley
with his family at: Address: Frog End Great Wilbraham Cambridgeshire
and
DAWSON, Sidney Edward Son Single M 1 1910 [born] Norfolk Norwich
with his family at: Address: 12 Old Yeast Yard Norwich

Was your Sydney born in the Isle of Man?

Was Edith Mona? Or just died there? If you give me her maiden name I'll have a look on the census.

Sue