OK so done that, apache logs show many of these:
{"level":"error","ts":1683883942.1298702,"logger":"http.acme_client","msg":"challenge failed","identifier":"subdomain.domain.com","challenge_type":"tls-alpn-01","problem":{"type":"urn:ietf:params:acme:error:connection","title":"","detail":"211.148.52.142: Connection reset by peer","instance":"","subproblems":[]}}
{"level":"error","ts":1683883942.1299212,"logger":"http.acme_client","msg":"validating authorization","identifier":"subdomain.domain.com","problem":{"type":"urn:ietf:params:acme:error:connection","title":"","detail":"211.148.52.142: Connection reset by peer","instance":"","subproblems":[]},"order":"https://acme-staging-v02.api.letsencrypt.org/acme/order/101863034/8706682724","attempt":1,"max_attempts":3}
{"level":"error","ts":1683883942.129954,"logger":"tls.obtain","msg":"could not get certificate from issuer","identifier":"subdomain.domain.com","issuer":"acme-v02.api.letsencrypt.org-directory","error":"HTTP 400 urn:ietf:params:acme:error:connection - 211.148.52.142: Connection reset by peer"}
{"level":"error","ts":1683883942.1299856,"logger":"tls.obtain","msg":"will retry","error":"[subdomain.domain.com] Obtain: [subdomain.domain.com] solving challenge: subdomain.domain.com: [subdomain.domain.com] authorization failed: HTTP 400 urn:ietf:params:acme:error:connection - 211.148.52.142: Connection reset by peer (ca=https://acme-staging-v02.api.letsencrypt.org/directory)","attempt":9,"retrying_in":1800,"elapsed":5414.863948805,"max_duration":2592000}
EDIT: Iāve now tried some other domains and IP addresses with no joy. Strangely though the 8443 port now seems to have worked. I get a valid certificate and the landing page saying the login is blocked because nextcloud is running.