I don't know about your particular situation, but the majority of schools don't have 'catchment areas', but calculate the distance from your house to the school. That is generally measured in a straight line (although some authorities do it differently - the admissions booklet should explain it).
That means that the closer you live to a school, the more likely you are to get in (assuming that 'distance' is in fact one of their admissons rules - you'd need to check that).
However, how close you'd need to live to a school to get in would depend on what sort of demand for the places there is in that given year. So if loads of people applied, you would have to live closer to the school to get a place. If fewer people applied, more of the people who lived a bit further out from the school would get places. So the size of the 'catchment area' would change every year.
This can also be affected by siblings of existing pupils, who are usually given priority over people living close to the school. If there are lots of siblings, only people living very close to the school are likely to get places. If there aren't many siblings, people from further away would be more likely to get in. The siblings issue usually applies to primary schools rather than nurseries.
However, some schools do have catchment areas. Look at the admission rules for the school (you will be able to find them on your local authority's website) and it will tell you whether that school uses catchment areas / distance / completely different admission rules.
And DecorHate is absolutely right - usually there is no advantage to having gone to a school's nursery when it comes to getting a place at Reception. Check the primary school's admissions rules - unless they state that children from the linked nursery as one of their admissions rules, it would be illegal for them to give preference to children who had been to their nursery.
Give the school(s) a ring and ask them to explain the admissions process to you. You can also contact your local authority and ask to speak to someone in their admissions team whose job it is to help parents understand and go through the process.
HTH.