It sounds to me as if your IP is talking to the wrong people at Northern Rock. If the proposal has merit, and represents the very best offer available, then they will usually accept it - but saying that the debts could be repaid over a 30 year period is not usually a criteria point.
The together loan product is not actually part of a 125% mortgage, but is an unsecured element of lending which was advanced at the same time as the main mortgage at the same interest rate - usually a lot lower than standard unsecured loan rates. So the loan was over a very long period - giving Northern Rock a long term return on their investment, but allowing their customer to benefit from reduced monthly repayments. Whether these products were wise is mute point, suffice to say that it is no longer an available product in today's market.
It may be beneficial to find out exactly why they have rejected, and see if your IP feels able to negotiate a settlement. If not, you may wish to seek a second opinion, nefore considering bankruptcy or a never ending DMP.
Regards, Melanie Giles, Insolvency Practitioner