What is the whenbuilt of the Brighton built Blandford Forum with a withdrawn of september 1964?
CREATE TABLE "british_railways_batch" (
  "br_no" real,
  "name" text,
  "builder" text,
  "whenbuilt" text,
  "withdrawn" text
);
SELECT "whenbuilt" FROM "british_railways_batch" WHERE "builder"='brighton' AND "withdrawn"='september 1964' AND "name"='blandford forum';