Add column with a sum total to crosstab() query in PostgreSQL 9.0
Proper crosstab query
First of all, the presented query would not work while you still have char(n) in your table definition - like we discussed under your previous question. You would need to convert to text or varchar first.
This works with your current table definition:
SELECT * FROM crosstab(
   'SELECT postcode_nospace_::text, ageband::text, count(ageband) AS ct
    FROM   adult_social_care.activities_in_localities_asc
    GROUP  BY 1, 2
    ORDER  BY 1'

   ,$$VALUES ('18-24'::text), ('25-34'), ('35-44'), ('45-54'), ('55-64')
           , ('65-74'), ('75-84'), ('85-94'), ('95 AND OVER')$$
   )
AS t("postcode" text
   , "18-24" bigint, "25-34" bigint,"35-44" bigint, "45-54" bigint, "55-64" bigint
   , "65-74" bigint, "75-84" bigint, "85-94" bigint, "95 AND OVER" bigint);
I am also using bigint instead of numeric to save another unnecessary type conversion (count() returns bigint).
Detailed explanation:

PostgreSQL Crosstab Query

Better table definition
But better ALTER the data type in your table to text like @dezso suggested. Or, if you want to keep the length limit, add a CHECK constraint or use varchar(n). Never use char(n).
ALTER TABLE activities_in_localities_asc
   ALTER COLUMN ageband TYPE varchar(12)
 , ALTER COLUMN postcode_nospace_ TYPE varchar(8);

 ALTER TABLE gazetteers_and_addresses.unit_postcode
   ALTER COLUMN pc_area TYPE varchar(10);

Then you don't need to cast original column values any more:
SELECT * FROM crosstab(
   'SELECT postcode_nospace_, ageband, count(ageband)
    ...

Ideally, though, ageband should be an enum or (my preference) an ID referencing a lookup table, not plain, error-prone text ... key word: normalization.
Add total count
Building on your table, but with text or varchar() columns.
Wrap your original query into a CTE and call the the result twice in a UNION query - the 2nd call adds totals per postcode:
SELECT * FROM crosstab(
  $$WITH cte AS (
      SELECT postcode_nospace_, ageband, count(ageband) AS ct
      FROM   adult_social_care.activities_in_localities_asc  
      GROUP  BY 1, 2
      )
   TABLE  cte  -- original results
   UNION ALL   -- add total per postcode
   SELECT postcode_nospace_, 'total' AS ageband, sum(ct) AS ct
   FROM   cte
   GROUP  BY 1
   ORDER  BY 1$$  -- dollar-quotes to include single quotes easily

   ,$$VALUES ('18-24'::text), ('25-34'), ('35-44'), ('45-54'), ('55-64')
           , ('65-74'), ('75-84'), ('85-94'), ('95 AND OVER'), ('total')$$
   )
AS t("postcode" text
   , "18-24" bigint, "25-34" bigint,"35-44" bigint, "45-54" bigint, "55-64" bigint
   , "65-74" bigint, "75-84" bigint, "85-94" bigint, "95 AND OVER" bigint, "total" bigint);
About the TABLE command:

Is there a shortcut for SELECT * FROM in psql?

Related CTE examples:

Combine results of two queries, where the second relies on the first
Get n grouped categories and sum others into one