Best way to get tickets for the Leonardo da Vinci (London)

Hey Londoners I will be in London for the weekend of Protected content and would love to see the Leonardo da Vinci exhibition at the National Gallery however, it seems that it is a hassle to be able to get a ticket. The presale tickets are gone in November and some websites sell some for as high as £150 which is crazy ! Does anyone has experience about getting tickets on the very day from the national gallery ? I have heard that the waiting time is too long and you should be there well in advance but how well in advance is the question ! Will 3 hours do ? And is it possible to buy seveal ticekts for one person ?
Cheers,

Funda