Further to Dee Sa's answer about the times for visa applications at the Indian High Commission:
The doors open at 8.30am. You have up until noon to be allocated a queue number. (You might be able to speed things up a bit by downloading the visa application form and completing it in advance).
Confirmation of those times, together with a link to download the form, can be found here:
http://www.hcilondon.net/visa/visa-application .html
Check that there is 6 months validity left on your passport, otherwise you won't be issued with a visa. As Dee Sa says, remember to take 2 passport photographs and �30 in cash.
Although (as Dee Sa states) Holborn station is fairly close to the High Commission, the nearest station is actually Temple. (From Liverpool Street, take an eastbound Circle Line train for 7 stops).
On leaving the station at Temple, turn left and then follow one of the streets which goes slightly uphill, away from the river (This will be either Arundel Street or Surrey Street). This will bring you onto the Strand, where you should turn left. A short distance along, you'll see a BBC shop on the right hand side. This forms part of the BBC World Service building, Bush House. Go past Bush House and turn right. The entrance to the High Commission, for visa applicants, is on your right.
Chris