Henry Cavendish does not have open days, they just arrange tours for interested parents in small groups which last around 30-40 mins. If you are trying to call them to arrange a date, I would suggest avoiding 8.45am to 9.30am, 11.45am to 12.45 and 3pm to 4pm as the office staff are usually busy dealing with parents in person during those times. If you are still having trouble getting through and you are able to do so (I appreciate that you may work), I would suggest dropping in during school hours to talk to them to arrange a time. For what it is worth, I have two daughters there, one in Year 6 and one in Year 1 and they and I are very happy with the school, but you definitely need to go and see the school yourself as your children and/or proirties in a school may well be different to mine!
BTW, Henry Cavendish is in Lambeth, and I think that Telferscot may well be too.