Anyone know what Barclaycard are like with limit requests?

binaryuniverse

My barclaycard currently has a 2300 limit. At the mo I have around £1800 worth of debt on it at 6.9%. My 3.9% for 6 months deal with M&S will run out in February and its highly unlikely I'll be accepted for a new card (I already have 6!). So my plan is to transfer over the M&S card to Barclays for another 6.9% offer (provided they'll let me do it, of course.) However, I've worked out, after 4 months or so payments + interest to each card, that I'll need just under 3500. A 65% increase.
I've always paid on time and never missed a payment, but can only afford about the minimum payment each month, so they'll still be making money off me. Just wondering if anyones had any experience of this?