Late September is THE best time to visit the South West. Generally good weather, warm water but none of the crowds. Cheaper too.
I can recommend Porthleven on the south coast – lively pubs, good restaurants and great walking and swimming nearby. Still a working harbour so not as gentrified or crowded as other places (yet!) and a great base to explore both the Lizard and Penwith. Kynance Cove, Porthcurno / Porthchapel and Sennen all fab beaches withing striking distance.
If you want quiet then the Roseland Peninsular is ideal- take a look around Portscatho area – Towan and Porthbeor beach both lovely and almost deserted for much of the year.
St Just / Cape Cornwall area has some stunning coastal scenery and walking – Priests Cove below the Cape itself is a lovely spot for rock pools at low tide.
St Ives fails on your criteria and gets rammed in the summer but worth a visit out of season – some great places to eat. Porthminster beach cafe is particularly good and the beaches are great.
St Agnes area is lovely. Further north then Trebarwith Strand and Crackington Haven are both favourites of ours.
If you are after a cottage then the following link will take you to some places the company I work for has available for the end of September, all close to sandy beaches.
http://bit.ly/1Llf9mo
If you want any further advice, then please feel free to PM me.
PS – Please don’t tar all surfers with the same brush. A lot of us surf down here – it’s a way of life for many and part of the reason they live here. The obnoxious minority you refer to are mainly wannabes and annoy us as much as they do non surfers! And don’t be put off trying it yourself, but as the advice goes, careful, it could change everything!
Hope you have a great trip wherever you end up.