Try using a sub query to make it one query.
SELECT PLATE_NAME, DATE_SYNTHESIS_BEGAN, CHEMIST, AVERAGE_PLATE_MASS, BATCH_NUMBER, EVERANALYSED,
FROM SYNTHESIS_PLATES SP,
(
select sc.synthesis_plate_id,
count(sc.SYNTHESIS_PLATE_ID) EVERANALYSED
from
synthesised_compounds sc, anal_plates_compounds...