Will I be able to have two broadband suppliers?
Yes as you will have Sky by the phone line (ADSL) and NTL via fibreoptic cable.
Sky are requesting a MAC code. Do they need this because I have been on a cable broadband?
No as far as ADSL is concerned you are a 'new' customer,
Does Sky support Outlook express and what other differences will I notice? Is MSN supported as the kids spent hours chatting freely on this facility
Yes and Yes.
If you can get a deal with NTL it may be worth staying. With cable if you pay for a 10Mbps service that's what you'll get (or very close to it). With any ADSL service it's 'UP TO....' which could be 8 meg or 4 or 1/2 depending on the time of day, phone line condition and length.
I have seen NTL customers getting access for less than £10/month.
On a personal note, I'm been trying to join Sky BB for over a month now and my date is the 5th DEC. This is the third date I've been given and the fourth time I've given my details to sign up !?!?!?