London elects says hold up with mayoral result is because 2 boxes of ballot papers from Brent and harrow weren't manually entered in system.
In a statement they say:
We have results in from 13 of the 14 constituencies in the Mayor of London and London Assembly election.
The final constituency is Brent & Harrow.
Two batches went to storage without some ballot papers being manually entered as required. Manual entry is required when a scanner cannot read a ballot paper – for example if a ballot paper is damaged. It is not an issue with the scanners.
The issue was identified during the verification stage. These two batches are being re-processed. To make this happen as quickly as possible we have separated out into several smaller batches.
We will declare as soon as possible but it is obviously important that every vote is counted.
Boris Johnson appears to have a strong lead, as counting begins in the London Mayoral election.
Ballot papers have begun to be processed for five of the 14 constituencies, Bexley and Bromley, Croydon and Sutton, Greenwich and Lewisham, Havering and Redbridge, and Merton and Wandsworth.
The chart below, published by London Elects (the body that organises the elections), shows the percentage of votes for each candidate in the ballot papers counted so far.
Detailed results can be found on their website.