Ant,

most common things to look for on a STTNG are the 3 green kidney shaped inserts, on the centre of the playfield, almost always have some sort of crazing,cracking or something wrong with them, the neutral zone hole is usually beat because of the gun launchers and these plastics ( link below)  are usually broken.

http://www.pinballheaven.co.uk/shop/product_info.php?products_id=535&osCsid=45e82aa831e4245466e9aa34d468c360

Also this game has alot of opto switches so check to see if they are all functioning.

hope this helps...