PostGIS Update parse error – invalid geometry

I’m trying to update geometry table using the following command but get

parse error – invalid  geometry
HINT: “MULTIPOINT(“ <--  parse error at position 11 within geometry

    Update geo_table
    Set geom =ST_ConvexHull(
                                ST_GeomFromText('MULTIPOINT(co_ord_string)', 4326));
    WHERE 
    Id =1234;

originally I created this data with a perl script something like

     insert into geo_table (stuff,date, co_ord_string,geom) values (‘Hello’,’02-07-2018 00:00:00’, T_ConvexHull(ST_GeomFromText('MULTIPOINT($co_ord_string)', 4326));

The co_ord_string is a series of long lat points separated by a comma.
These don’t create perfect polygons as they may contain internal points but using ST_ConvexHull a polygon is created.

Only by viewing the data in QGIS I can see there are errors in the original coordinates that need manually editing (Missing minus signs some points or other issues ) and the geometry recreating.

What is the correct syntax?

UPDATE geo_table
SET geom =ST_ConvexHull(
 ST_GeomFromText('MULTIPOINT(' || co_ord_string|| ')', 4326)
);
WHERE Id =1234;