Martin, there was a regular pair in a small patch of non reserve land at the eastern edge of Cors Ian next to a pasture grazed by beef cattle so worth sitting up the gorse hill to observe any comings and goings.
On one visit in late June I was on the reserve when I heard curlews alarming strongly and turned to see 8 flying around over a field being cut for silage uphill on the south side; I tried to tell myself that the young had escaped but it was much more likely given the date that they hadn't fledged and were in danger from the mower.
The farm at the junction of the lane with the LLedrod road also had breeding curlew 8 years ago and I did approach the farmer to warn him they were there but he didn't show much enthusiasm and later I noted that the rushes had been mowed; this was before the latest plan for farmers to cut rush. Again it would be worth having a look from the lane across to a bank and some stone remains. Young curlew have been seen near there on Swyddffynon common.
For anyone visiting Cors Ian: be mindful if going off piste as the water voles territory extends away from the stream for some distance!