Martin,
The London Electoral Roll are a complete mess, if anything Ancestry has made a worse job of Transcribing them than the 1911 Census. Have a look at the Ancestry UK Blog
http://blogs.ancestry.com/uk/2012/01/11/130-years-of-london-...Basically they have used OCR technology to scan the LMA supplied Microfilms. They have then indexed the results using all sorts of Boroughs and Parliamentary Divisions that did not exist at the time. There is no consistancy to what has been done so a roll from 1880 may say, for example, its for Finchley, London and the 1936 roll says its for Finchley, Barnet and the 1937 one is completely wrong and says its for Hornsey, Haringey.
Title pages have gone through the scanning process and the scanner has tried to read voters names into the text resulting in gobbledegook. The OCR process has skipped names on the images, so some people are missing, and read in some street names as people. Even clearly visable names have been badly mangled. Nothing appears to have been checked as the errors are so obvious that if it had been checked this collection would surely never have been released.
There are supposed to be 139 million records in this collect, but just how many are correctly transcribed is a good question. The mistranscribed 1936 and 1937 rolls for Finchley contain approximately 12,000 records and every single one is wrong!
If you want an idea of what has been transcribed in this batch follow the link below
http://search.ancestry.co.uk/search/db.aspx?dbid=1795 you can get dropdowns for the Boroughs/Counties, Parliamentary Divisions and Years that have been transcribed. But remember that many of the rolls are misindexed. Hardly anything is recorded for the counties of Middlesex and Surrey, instead Ancestry has tried to allocate the historic rolls to the current 32 London Boroughs which were only created on 1st April 1965 which is the latest date of this collection. I know its not easy to be historically correct as some areas have moved from County (Middlesex, Surrey, Kent, Essex and Hertfordshire) into the City of London and then into the GLC and are now London Boroughs but every roll that I have looked at cleary stated the correct information if only a human had read it and not a machine!
In an e-mail from Customer Services it was indicated to me that Ancestry hope to double the number of records in this collection by the "Fall". I hope they make a better job of this second batch.