Triggers / Referencing Non-updated Fields?

I'm having a bit of a problem getting the syntax of a trigger right. Unfortunately, I have no DBAs locally, I use some third party software, and for reasons beyond my understanding, I have been told to use triggers, and not stored procedures, so I'm running with it.

The set up:

STRUCTURES table: contains several columns, one of which is the unique ID column.
ASSAY table: contains several rows, also with the same ID column, but can have more than one row per ID (several assays per compound). One column is XC_ASSAYS.

The idea of the trigger is basically: When a row in the ASSAYS table is updated, pull out the ID of the row, then calculate the average of the XC_ASSAYS columns for those rows, and report it to the STRUCTURES.XC_ASSAY column for that row ID.

My best attempt thus far results in compilation errors.

CREATE TRIGGER INHIB_W_ALA_TR
AFTER INSERT OR UPDATE ON ASSAYS
FOR EACH ROW
BEGIN
UPDATE STRUCTURES SET XC_ASSAY = (SELECT AVG(XC_ASSAY) FROM ASSAYS WHERE ASSAYS.ID = :NEW.ID) WHERE STRUCTURES.ID = :NEW.ID
END;
/

The resulting errors are:
LINE/COL ERROR
-------- -----------------------------------------------------------------
2/1 PL/SQL: SQL Statement ignored
2/190 PL/SQL: ORA-00933: SQL command not properly ended
12/0 PLS-00103: Encountered the symbol "end-of-file" when expecting

[code]...

I don't understand some of the errors, such as why line 2 SQL is ignored (it seems correct?), or I'm supposed to properly terminate the trigger (I've read ; and /, but I'm getting the end-of-file errors when I do so). Tried shuffling syntax and ' or " around - and I can't get it.The body SQL works when I replace :NEW.ID with an actual variable (such as 'NMP12'), but I'm not sure how to pass the ID variable from the updated row into the body. The ID is not updated, but other columns are.