who do I contact about going on the waiting list/s (does it depend on individual schools). If applying to schools in different borough, would you need to contact both LEAs and respective schools.
You will get full details of how to apply for waiting lists in your allocation letter. Your home LA should be the first point of contact, although sometimes things have been known to go slightly pear-shaped between LAs after the initial allocation process, so it can be wise to check with the other LA(s) that your child is definitely on the waiting list for each school, and get it in writing
Could anyone offer a brief outline as to how a waiting list works?
There are generally fixed dates, around 6 weeks apart, when further admission rounds take place, so roughly April 15th, June 1st, etc. The biggest shake-down is in the second round. After that it is increasingly unlikely that your child will be offered a place.
Also are all school's obliged to tell you where exactly on the list ones DC is.
Yes, but bear in mind that your child can move up or down the list. If someone moves into the area and qualifies, and they are ahead of you under the admissions criteria your child could be "gazumped". That doesn't tend to be much a problem for grammars, but it can present a problem for other, non-selective schools. I have known someone go from 1st to 4th on a waiting list within a couple of days, and then gain a place a couple of days later! (That was in July when "formal admission rounds" often get thrown out with the bathwater.)
What would you think a rough number your child would be very unlikely to get a place? Something like 20+ maybe? Again could the answer to this be quite specific to individual schools.
Very much specific to each school, and it depends on lots of factors, although I would say that after the second round of admissions, anything below 5th is unlikely to gain a place at most grammars.