Changing PTR record

[#1]

Is it possible for PlusNet to change the PTR record for an IP on PN Business: if so, how?

[#2]

Indeed you can, provided you have a static IP with us.
In order to do this, you must create a forward A record for the value you want and point it to the IP.
So to have the PTR as mail.domain.co.uk. You need to have an A record for this.
Once done and is operational in DNS, raise a Support ticket confirming the IP and value you want.