Hi,
We had a similar issue and my wife had taken her IELTS SELT test in 2013, used the same cert for further leave and then used it for ILR, which she got 18.02.2019. Please check if her test centre is in the link.
However, I found the below information and used this in my cover letter:
"My IELTS English test certificate which was initially used for entry clearance as a spouse into the UK, and subsequently for further leave to remain in the same category in 2016. The English test result would be accepted under paragraph 32D of Appendix FM-SE as it had been accepted as part of a successful previous application.
The IELTS SELT (for all IELTS facets) test centre number IN855 is valid on the latest list of providers at the below link:
https://assets.publishing.service.gov.u ... ebsite.pdf