This is from bt.com, I hope it answers you query.

Presentation Number Calling Line Identity (PN CLI)
Presentation Number (PN) is a BT service that will allow you to specify a telephone number for your Calling Line Identity (CLI) for outgoing calls, which may be different to your usual telephone number. The specified PN CLI will be displayed on the called customers' Caller Display equipment if available, or used in conjunction with the 1471 service where calls terminate on analogue lines, to return calls to you.

You may require this because your business has separate incoming and outgoing lines for effective traffic management and resilience, or to direct return calls to a more appropriate number, for example a central helpdesk. Without PN, the only option available to you is to "withhold" your CLI if it is unsuitable for return calls. However, this can put off called customers who appear to be increasingly reluctant to accept and respond to "anonymous" calls.

This situation is becoming more common due to the availability of a service called Anonymous Call Reject (ACR) which was introduced by most network operators in March 2000.

ACR blocks incoming calls with CLI "withheld" and returns a recorded message. This adversely affects any businesses that withhold their number. PN CLI is intended to help overcome this problem.

It  is listed under Digital Select Services and you can read all about it here.

That's useful . . . I could do with one of those.
Do they do them in Home type versions as well  ?

Or does anybody know an to change it over (& back) for short periods ?

I got threatened yesterday 'cos I accidently rang someone at 2am and they get up for work at 4pm !! He was a scot from Glasgow and he rang me back in the evening offering me a permanent end !! I was very apologetic and graciously declined . . .

He seemed happy at that but should I be worried !?  

BTW Thanks Paul
Following that link to read about PN and ACR, I just rang up & got ACR set up on my phone. Blocking didn't work on the cold calling, so maybe ACR will help.