Hi there,
Yes it could cause a problem for you, I was in a similar situation, however we got around it, Is it only 400 to the debt collectors, if so you can contact them and offer to pay it in full, then when you have paid it ask for a letter stating the the amount has been paid and there is no further problems with Ulster bank, also ask ulster bank to send you a letter confirming they are happy with the end result,
The next thing you need to do is contact the bank you are applying for your mortgage with and tell them the situation, Do not try and avoid the situation with them, as they will find out and then ask what the problem was, if you tell them first then it will stand in your favour,
I would not worry to much if you can pay it off, you can also contact Ulster bank and request it to be taken off the Icb, they have the authority to do this under certain circustances as well, and if you clear up the loan and they see that you have genuine enough they might look in your favour, or also if you know anyone who works for any bank they could pull a few strokes and get you off it,