Those nice people at London Elects have been kind enough to make ward level data from the 2008 Mayoral / LA elections available, and turnout and vote share maps having been done elsewhere I've had a go at doing spoilt ballots, council by council.
Black (hello and welcome, Tower Hamlets) represents 3.2% of all first choice ballots being ineligible through mismarking, no mark etc. Dark grey (Ealing, Brent, Newham and Barking and Dagenham) represents 2.1-3.1% ineligibility and light grey 1.1-2.0%. Very light grey - Richmond - managed a 99% success rate and the City of London 99.6%.
Ward level data shows that the very worst figure is a 7.08% vote failure in Alperton, a ward in Brent. if Wikipedia is to be believed, Alperton is 75% ethnic Indian and 10% ethnic Sinhalese/Tamil, so it would seem that language might be a problem. Also at or around 1 in 20 are are Southall Green (Ealing, 5.9%), Spitalfields & Banglatown (Tower Hamlets, 5.5%), St Dunstan`s & Stepney Green (Tower Hamlets, 5.1%) Whitechapel (Tower Hamlets, 4.9%), Norwood Green (Ealing, 4.9%), Larkhall (Lambeth, 4.9%) and Kenton East (Harrow, 4.8%).
At the other end of the scale, two out of 1,312 City of London postal voters made a hash of it (0.15%), while the most talented polling station visitors were to be found in Kensington & Chelsea's Royal Hospital ward where 99.43% got it right. Nice that the Chelsea Pensioners are keeping good company, and doubtless providing it themselves.
Digging a little bit deeper, Alperton leads for blank ballots - 4.4% and while all of the electors of Bromley -Biggin Hill found something to mark. Identifying oneself on the ballot paper seems to be a Lambeth vice, at 2.8% in Larkhall and 1.4% in Oval. Over voting was big in Plaistow South (Newham) - 3.3%, Southall Green (Ealing) - 3.2% and Tottenham Hale (Haringey) - 3.1%