Can anyone recommend a good postcode distance tool?

[VivaLeBeaver]

Admissions authorithy have got my distance to the school as 13.01 miles and cutoff is 12.6 miles this year. However they've used the main road to calculate that and there is a country lane which is a shorter distance. School's admission policy states its by road distance on any public highway so I'm trying to see how low I can get it.

You can try Google maps or similar but that won't be as accurate as the system your LA use. I would try contacting them and asking if they can give you the distance using your route.

If they have not used the shortest route that is a very good basis for an appeal. The appeal panel should get them to remeasure using your route. Note, however, that they will also have to remeasure any other applicants where they have used the main road rather than this country lane to see if they are also affected. You will only get a place on this basis if the remeasurements show that you should have been offered a place initially.

AS PRH says you need to get the admission office at the LA to confirm exactly what route was used and the distance for both. The system they will (should) be using is easily capable of going down to less then 0.001 miles, so I am a little surprised that you are suggesting that they have ignored a country public road. The computer normally checks out alternates and chooses the shortest, which may imply a problem with their system. If that is the case then the LA will have to remeasure all distances and that might not improve your situation.
You could try using google and forcing it to do the two separate routes to see what the difference is, but it is as easy and probably as accurate to just do the two journeys in the car and see what the difference is. If it less than 0.51 then it is not going to make any immediate difference any way.
You probably have more chance given the distances involved at appeal, especially if it is your nearest school.

We used something similar to this www.goodrunguide.co.uk/RoutePlanner.asp
when we were disputing the distance from our gate to school gate with our LA. You can actually go along the individual route and it gives you the distance travelled.

OK, I've emailed the admissions office asking them to clarify which route they used.

You need to double check how the distance is measured.
Surrey [[http://findaschool.surreycc.gov.uk/
... measure the distance from home to school is by straight line, from the address point of the pupils house, as set out by Ordnance Survey, to the nearest school gate available for pupils to use.]]

Have you tried walkit.com? Not sure if it would help but maybe.