Credit card shuffle and minimum payment.

Rustic_Ron

Help me make up my mind about shuffling credit card balances.
I have been moving from cards to cards for a few years now. I mostly only pay the minimum payment on 3 cards. 2 x 0% cards (£3k each) and I pay approx £35 on each. I also have an mbna 5.9% for life card(£4.5k) and pay £68 p/m. I have used most card companies and now due to having lower income coming in I cannot get a card with enough limit to combine. So I am running out of forward options. I always close my previous card accounts when a transfer is complete just in case I need to go back to that card company in the future.
I also have a loan with tesco(6.2%) and owe £6k and 5 yrs still to pay.
I am happy with the interest rates on my debt but as I say with low earnings and limited card companies I am concerned that I will soon be reverted to 18.9% or more on two of them.
I tried to add my debts to my unsecured tesco loan but although I have good history they will not lend me any more due to my earnings and current commitments.
I have been told to steer clear of adding the total £16k to my nationwide mortgage (15 yr @ £148p/m). But in my head it seems to be best option .
Why ?? well My debts will be with one company and I have worked it out that if I pay an extra £30 per month back then it will bring down the repayment period quite swiftly..The rate of interest is 3.99% variable. Even if rates jump 3% it will still be cheaper than a bank loan!! in which I cannot get.

So I would welcome your advice or ideas.

Based upon my current income of £12k I am unable to overpay on my cards but I should be able to on the nationwide further advance option..
If my income was higher (Like it was 2 yrs ago) then I could slightly overpay. Thanks all.