I have only done a few track days, some track day gods may be able to give you a bit more advice but I'll answer from my limited experience.
How long will I get to ride during the day?
Normally 5 or 6 20 minute sessions. Doesn't sound a lot but you will be knackered!
Will they kick me off if they think a sprocket is too worn or something?
If your bike's condition is likley to damage the track or another rider you may get black flagged. A worn sprocket will be fine, but you will be working your bike harder than normal so probably best to have it in tip top condition. A certain forum member on here did a whole day at Silverstone on tyres squarer than Spongebob's pants and didn't have any problems.
Do I need both parts of my driving license?
Yes, although I have never had them check both.
Can I bring friends to watch for free?
Yes
Are they a decent company?
trackdays.co.uk are just a booking agent, the particular day will be ran by easytrack, I have yet to use them but a few people on here say they are the best company running track days at the moment.
http://www.easytrack.co.uk/bike-trac...nd-march-2012/
And, would anybody like to come?
Brands is a bit far away for me as I'd have to ride down there in the morning and ride back at night, and I am working that day but I too have some vouchers that need using so I am tempted.
Have a read of this, should tell you all you need to know
http://www.easytrack.co.uk/novice-gu...ke-track-days/