Andy,

You sir are a scholar and a gentleman of the highest order.

I made the changes you suggested and here is the result

; <<>> DiG 9.16.27-Debian <<>> 35.84.119.85.in-addr.arpa PTR
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 60747
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 3, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;35.84.119.85.in-addr.arpa. IN PTR

;; ANSWER SECTION:
35.84.119.85.in-addr.arpa. 84462 IN CNAME 35.35-32.84.119.85.in-addr.arpa.
35.35-32.84.119.85.in-addr.arpa. 600 IN PTR keynesmail.com.

My new zonefile:-

$ttl 600
; $ORIGIN 35-32.84.119.85.in-addr.arpa.
35-32.84.119.85.in-addr.arpa. IN SOA ns3.keiths-place.co.uk. keithwilliamsnp.gmail.com. (
2022041211
600
300
1200
900 )
NS ns3.keiths-place.co.uk.
NS ns2.keiths.place.co.uk.
NS ns1.keiths-place.co.uk.
35.35-32.84.119.85.in-addr.arpa. IN PTR keynesmail.com.

I will now return the TTLs etc to a more sensible number, and all will be set.

Thanks so much for the help, everyone.

Keith