Hi Des, that's a good area to bird in Sept, Pwllheli itself has a nice area of mud, i've seen Curlew Sands, Whimbrel, Greenshank etc at that time of the year. Nearby the Porthmadog area is even better, i've had Osprey, Hobby, Curlew Sand, Med Gull etc there in Sept with the chance of Whooper, Red Kite, Otter (after dark!) etc. Black Rock Sands has a few Velvet scoter, Scaup, Eider; Criccieth can be good in strong SW winds for Little Gull, chance of a skua while Porth Meudwy valley west of Aberdaron could have potential September scarcities like Firecrest, Hippolais warblers, Turtle Dove etc., Foryd Bay to the north on the way to Caernarfon is a bird-fest, then you've got Anglesey with Malltraeth, Point Lynas, Cemlyn; best bet is to check the Northwalesbirding website or get the Roberts and Davies site guide - be careful with the Helm WTWB some of the entries are sheer fantasy!
Hope you have a great trip.