Martin Brown wrote:

mail.ourdomainname is probably an alias for one (or more) of their
physical servers. One level of indirection won't hurt at all.

mail.ourdomain.co.uk is a "CNAME" for the apex domain ourdomain.co.uk

ourdomain.co.uk has an "MX" record of mail.bobserver03.co.uk

the host mail.bobserver03.co.uk and the apex domain ourdomain.co.uk
both have "A" records 5.77.60.195

obviously you can't use a CNAME in an MX record.