Javascript postcode radius app

also prepared to look at a fixed cost and am flexible on wages for quality.
I have a database of full uk postcodes with lat and lon. I need a javascript app that will allow me to enter a specific postcode, and a radius in km (or miles;not fussy) - the app would then output all the postcodes from the database within that radius.

So the app would have to
1) accept input of a UK post code
2) accept input of a radius from that postcode
3) calculate the position of the postcode from 1)
4) output every postcode in the database that was within the defined radius

ideally the output would be in CSV, but it's not a major issue so long as i can copy the results into notepad