MBNA are not the easiest creditor to deal with. However if you have made the best payment offer you can there is really little more you can do than continue to pay them so the debt management company you are using is doing the right thing. You should not try to pay them more as this will mean it is more difficult to maintain your other DMP payments and put the whole plan at risk.
Generally after hassling you for a while MBNA will pass your account to a debt collection company and they will normally be more sympathetic.
In terms of whether they will apply for a CCJ against you this would be unusual but it is possible. If you do receive a CCJ claim form through the post speak to your DM company about it straight away. Even though you are in a DMP it has to be completed and returned but if you are making your best effort to pay then the Court should accept your payment proposal. For more information about what to do if you receive a CCJ claim form see: How do I complete a CCJ Admission Form