Hi Peter
London historically and geographically is very complex so it is very important to be as precise in terminology as possible when you are talking about it. When you mention City of London parishes my first thought was you literally meant the parishes of the City of London itself which had/has over 100 historic churches (about 119 to be more precise pre 1812) just within the square mile of the City of London alone. Genuki lists them all for you.
http://homepages.gold.ac.uk/genuki/LND/parishes.html#parYour map shows the boundary of the square mile of the City of London, not the parish boundaries within it. It doesn't include as most maps don't, the part of Southwark south of the Thames which historically was also controlled by the City between 1550 and 1899 and known as the 'Ward of Bridge Without'. This was a part of Southwark but not the whole of Southwark.
http://en.wikipedia.org/wiki/City_of_LondonWithin presnt day London there are two cities - the City of London and the City of Westminster. If Croydon had its way there would be three. London today encompasses these two cities and much more besides.
As London is ever evolving I think you have to decide which time frame you are working from. Genuki again lists the historic Middlesex parishes
http://homepages.gold.ac.uk/genuki/MDX/ParishIndex.htmlwhich after about 1832 began to be increasingly subdivided into further 'daughter' parishes. See for instance Genuki's page on Stepney churches
http://homepages.gold.ac.uk/genuki/MDX/Stepney/churches.htmMiddlesex 'parishes' surrounded the City to the west, north and east and gradually became accepted as part of London. By 1898 you have the establishment of the LCC - London County Council (1889-1965) which officially incorporated into London all the Middlesex, Essex, Kent and Surrey parishes it had absorbed in practical terms already. From 1965 onwards this part of London was termed 'inner London' and further massive areas of the surrounding counties were added which are now known as 'outer London'.
The best parish county maps are found in the Phillimore Atlas and Index of Parish Registers. It is not cheap by any means (you can get them for half price on Amazon) but absolutely invaluable. Their maps and lists of parishes stop at 1832. If you are looking at the historic Middlesex parishes I certainly wouldn't go past that date. (An example of a Phillimore map - Kent)
http://globalgenealogy.com/countries/england/general/maps/parish-maps/images/114a.jpgMy historical knowledge of London wards is more flaky. I believe there were about 26 wards within the City of London itself which elected Aldermen.
http://en.wikipedia.org/wiki/Template:City_of_London_wardsBy 1889 with the LCC there were 118 councillors (4 from the city and 2 each from the 57 electoral divisions). I haven't a clue how many Middlesex wards there were before the formation of the LCC.
So all in all I do think you really need to decide your historic cut off period and what boundaries you are drawing - the City of London boundary itself plus historic Middlesex parishes surrounding the City? (even that will get very fiddly just around the city - Phillimore does two maps for Middlesex, one of the larger parishes slightly further away from the City and an enlarged one of those surrounding it where the parishes in size are very small because the populations are/were so large).
This is a complex and ambitious project, as Pru says 'a task and a half' and some more really, and I do think there is a need for something which further helps people with 'inner London' geography, but it isn't easy to crack. Very interesting to see the possibilities of the technology with Google maps - I didn't realise you could do 'overlays' like that.
Regards
Valda