external score decline. what does this mean?

Thrugelmir

MBNA will be able to see that the balance you wish to transfer is already on a promotional rate. This possibly could be the reason behind the decline. 

Fighter1986

Help us out here.

You've been sent an MBNA card through the post but they've told you you were declined for the card?

Did they email you, write to you by letter, or tell you on screen at the end of the application that you were declined?

Have you tried to set up the card via their online banking to see if it is in fact open and working?

. They may have approved you and later the underwriters closed the account
. They may have initially declined you but later the underwriters opened the account

I've seen both happen in the past. 

If the account is currently open, you can use it to do the balance transfer. If it isn't, you don't need to do anything, as the account will automatically updated on your credit files to show open and then closed.

An opened and immediately closed account isn't likely to have a negative effect on your credit worthiness as long as it does show as closed on your credit files before you apply elsewhere.
Applying elsewhere soon after opening the MBNA card while it shows as open and active on your credit files might show a lender you're desperate for credit / accumulating lines of credit quickly and reduce your credit worthiness in their eyes.

Stage 1. is establishing for sure, whether or not the MBNA account is actually open and active.

Let us know when you've found out.