Lovely time of year to go. I stayed in the north east of the Island a couple of years ago in October and things were starting to close down there but when I visited my cousin who was staying in Palma Nova everything still seemed to be in operation.
I would imagine there will be a lot less people around than in the high summer weeks but the weather should still be reasonably hot, perhaps cooling down in the evenings.

September is exactly like May only a bit warmer, both day and night.

Thanks for your replies. Found flights for about £123 each with good flight times so might book those. Looking at hotels separately - anyone know any cheap sites to look at? Looking at something like Hotel Hawaii or Santa Lucia.

Travel Republic are usually ok. Try a google search for your hotel and see which sites come up then compare. Becareful though and check the agent on here first as sometimes the cheapest doesn't mean the most reliable

There is always a risk of showers in September - when ever we've gone in September we have always had a couple of showery / overcast days but they soon clear up plus the swimming pools are always a little bit cooler!