Fast times and slow starts

After a bit of advice about times from any experienced FLM runners
Only done 1 marathon before (NYC in 2001 as a sprightly young 28 year old)
and had not really pre-planned my finishing time (or anything to be honest).
Now a bit more organised, and dull, I'd like to write out splits for each mile to see if I am on track for a PB.
Main question really is how slow are the first few miles, and at what point can I expect to start running at a decent pace (probably about 8min miles, or a little bit slower)? I'm on the red start which, I believe, is the slowest one. Should I factor in, say 10 min miles for the first 2? Or less? And/or does the starts merging just after 3 miles also slow everyone down again?
Any help very gratefully received.
Cheers

It all depends which pen you start from, the lower the number, the nearer to the front you are, and hence will get held up less. You will not know your pen number until registration, and it will be based on the expected time on your entry form. If you entered a time of about 3 1/2 hours (approx 8 min pace) you will probably be in pen 3 or 4. I know this as it is the same as I have been for the past few years. Based on my timings, if you allow 3-4 mins over the first three miles, although these have always been from the blus start. I have not really noticed a hold up when the other starts merge together as for a while you are kept on seperate sides of a dual carriageway, and then gradually merge.

I started from the red start last year and didn't really notice much of a delay. Because most people go off a bit faster than they should, you can usually go at a reasonable pace as long as you're in the right pen (as Godwit said). If you're a bit worried you could try to sneak into a higher pen than you're meant to, but it could be seen as a bit selfish as the other runners around you would want to be going a bit faster.

Last year I was aiming for 8 min miles and started off going faster than this (I ran my first mile in about 7.30) because people around me were going that fast - though I was weaving through a few people, stupidly. So I reckon as long as you start in roughly the right pen you don't need to budget more than half a minute max for the first couple of miles, to allow for the start.

Cheers for the advice
Got my place through a charity, so can't remember if i had to fill in a projected time or not (mostly cause i was drunk when I filled out the Golden Bond form - always a good start)
If I did, hopefully I put 3hr45 or something, so should be close enough to what i want to do.
Cheers