Harpenden secondary school catchment

[Cirrus123]

Hi All
Just joined mumsnet (as a dad...hope thats ok) so here goes with my first thread!
Thanks for replies in advance.

We are moving into area from Bucks. Looking to buy house at some point but in meantime will rent. My son will be starting secondary school in September 2017 so trying to rent in close proximity to either Sir John Lawes or Roundwood.

Is it true that Roundwood catchment is larger than Sir John Lawes?

Looking at a couple of properties at the moment....one is 270m from SJL and another is 550m from Roundwood. My question is which is better option to get into one of the schools. Also in a couple of years my DD will be going to secondary.

Does the sibling rule only still apply if you still live in catchment? What if we buy a house in 18 months outside of,say the smaller Sir John Lawes catchment...what happens then. If Roundwood catchment larger then are we better off trying to get my DS into that school so if we buy elsewhere in Harpenden in future then more likely to still be in catchment so that both kids go to same school.

Apologies if that's a bit long winded. Quite confused currently!!

Thanks for your help.