I've begun to build an Excel spread sheet listing all the Sleeman births in England and Wales since September 1837.
That is when central record began, before that you have to rely on local records - hard work!
The registers cover 3 month periods, so the dates are not exact and only indicates names and the town. 
Initially the local Superintendent Registrar was responsible for the collection of records. It is estimated that 15% of births went unregistered. It was not until 1874 that the obligation to register births, deaths & marriages was transferred to the persons involved.  The record of Sleeman marriages and Sleeman deaths is also under construction.  Records will include the miss-spelling Sleman and Slemon.  The family name Slee was also widely evident in the records from 1837, but I have not recorded these.
It is possible to purchase a copy of the birth certificate to discover more details.  
The mother's maiden name is not recorded before September 1911.
Before March 1912, the wife's maiden name is not shown on the marriage certificate.
Only the age at death is shown on death certificates until June 1969, since when the date of birth is shown.

There must be Sleeman researchers who have done all this before, but now it's on the WEB!
The Sleeman "clan" have an excellent WEB site
old WEB site     old Forest of Dean family WEB site
[I used Microsoft Excel ver 4 as the data is very simple and you can process it for sorting by name, town, sex.]

There was a major concentration of the Sleeman family in:
Cornwall (Bodmin, Camelford, Helston, Liskeard, Redruth, St Austell, St Columb, St Germans, Truro) and close by in
West Devon (Barnstaple, Exeter Holsworthy, Launceston, Okehampton, Plymouth, Plypmton St Mary, Stoke Damerel, Tavistock).
There are a few entries for Sleemans living in Bristol and more at Clifton in Gloucestershire near Bristol,
and just over the River Severn at Westbury-on-Severn in Gloucestershire.  Ross-on-Wye in Herefordshire had a few.
Cardiff in South Wales has a few too.  The map shows the location of these towns.
There is also a chart showing distribution of Sleeman births, marriages & deaths by area using the squares on the map.
The odd one or two are listed for London in Chelsea, Kensignton, Marlybone and Shoreditch,etc. and to my surprise in Croydon.

A good introduction is given at the UK Public Records Office WEB site. The booklet "An introduction to Civil Registration" by Tom Wood (ISBN 1 872094 84 8 published by The Federation of Family History Societies) provides useful background on the UK registers.  Here is a Cornish Genealogy site worth a visit. Another booklet by Ray Wiggins "St Catherine's House Districts" (ISBN 0 9527098 0 5 published by J R Wiggins) lists the registration districts and their sub-districts:

Barnstaple + Bishops Tawton, Braunton, Coombmartin, Ilfracombe
Bodmin + Egloshayle, Lanlivery, St Mabin
Bristol
Camelford + Boscastle
Cardiff + Caerphilly, llantrisaint, St Nicholas, Whitchurch
Clifton + Ashley, Stapleton, Westbury
Exeter
Helston + Breage, Crowan, St Keverne, Wendron
Holsworthy + Black Torrington, Broadwoodwidger, Clawton, Milton Demerel
Launceston + Alternon, Northill, North Petherwin, St Stephens
Liskeard + Callington, Lerrin, Looe
Okehampton + Bratton Clovelly, Chagford, Hatherleigh, North Tawton
Plymouth
Plympton St Mary + Yealmpton
Redruth + Camborne, Gwennap, Illogan, Phillack
Ross + St Weonards, Sollershope
St Austell + Fowey, Grampound, Mevagissey
St Columb + Newlyn, Padstow
St Germans + Antony, Saltash
Stoke Damerel + Clowance, Morice, St Aubyn, Stoke, Tamor
Stratton + Kilkhampton, Week St Mary
Tavistock + Buckland-Monachorum, Calstock, Lifton, Milton Abbot
Truro + Kea, Kenwyn, Probus, St Agnes, St Clement, St Just
Westbury-on-Severn + Huntley, Newnham

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