There is a Catholic primary in Watford - St Catherine of Sienna in Garston (just north of Watford - basically part of Watford). And a Catholic secondary - St Michael's.
Admissions (to non Catholic schools) are done on straight line distance here. You need to use this calculator to find out how far you might live from differnet schools (bit hard if you don't yet live here though - perhaps to use when you are actually viewing houses!).
You can find out how many people applied to primaries and how many got places here:
2011 outturn report for Herts
2010 outturn report Herts
Presuming that you'd be applying under the rule about a school being your nearest school (rule 5) or the distance rule (rule 6), you need to look at the solumns labelled "5" and "6" to the right of the table to see how many people applied / got places under each of those rules. For some schools, eg Knutsford in Watford, even if it's your nearest school, you might not get in unless you really are very near.
However bear in mind that when it looks like 300+ people applied to a school but only 60 got places, that this includes everyone who put the school 1st, 2nd or 3rd in their choices, so quite a few of them would be not having that school as their first choice.
We have just applied for primaries in Watford starting this Sept so feel free to ask any other questions
. It took me ages to get my head round all this!
Would definitely be worth ringing possible schools to ask if they are likely to have a free place for year 3. This might narrow down your search quite considerably (in a good way hopefully)!
Then when you get to secondary there's some very good state options: Watford Boys, Watford Girls (both grammar and excellent results, academic), Parmiter's (very good results too), Queens (less good results, huge school but very decent too).
If Radlett is too expensive then St Albans definitely will be too.
Is there any reason why you don't want to live in Watford itself? Nice primaries include Knutsford, Nascot Wood (both Ofsted outstanding) and several other very decent ones (eg Parkgate). The train from Watford Junction is only 20mins straight to Euston - from most of the other places you will either have to travel into Watford (and pay £££ for parking) or the train will take longer. House prices really quite reasonable considering how close to London you are.