@CailinInUK
My experience was:
Modified natural FET with letrozole, antagonist added, cycle stopped, added menopur 75 units for two days and it got going again, got to transfer and had a chemical.
Next cycle same plan, good initial response with several follicles so consultant added the antagonist, once added all the follicles disappeared, low dose stims didn't help, lining dropped to 3mm, cycle was cancelled and I was put on the OCP in order to build the lining and get a bleed on the withdrawal. Consultant decided next time to remove letrozole.
Went to start with the OCP induced bleed, but had made my own follicle so had to trigger and await another bleed.
Next cycle was low dose stims, no letrozole and I refused the antagonist, was able to get a good lining and triggered, cycle was successful.
All the specialists said the antagonist doesn't shut down cycles but did acknowledge that it was happening in my case although they couldn't explain why. Someone I talk to said that the antagonist also interferes with implantation.