This is appalling. I'm so sorry this happened to you, I would have had a meltdown, honestly.
So, I'm not from the UK, which means that I have no idea what you might be entitled to or if what they did is even legal (wouldn't be legal here, trust me). However, having watched my mom for many years (a very sweet looking woman who does not take any bs from snooty people and has scared the living daylights out of more arrogant people in customer service than anyone I know), I would say you absolutely should demand some form of compensation.
I can only guess here but common sense leads me to think that you gave the SA the permission to debit your account ONCE. Not twice and certainly not three times. So whatever happened after they debited your account was obviously not done on purpose but certainly by someone who didn't know what they were doing. I'm sorry if I sound harsh but I have worked with a PDQ machine for years and yes, they can drive you nuts (especially when your boss is cheap and buys a machine that's broke all the time) but nobody who's worked there with me has ever had anything like this happen.
Again, I don't know if the system in the UK is so different that this can happen more easily but when you're handling sums like these .... I mean jeez. Better be safe than sorry and contact the customer to check in with them before you try it THREE times.
I digress, sorry.

Anyway, Mulberry is not handling this professionally at all. If this was "only" a matter of stress, I'd say you could possibly write it off as bad luck but you're out of money because of this since you had to take out a loan. Suggesting to waive your postage as an apology is a little ridiculous.
I really hope the ladies are right and this Rebecca girl can help you, that would be the best solution of course. *crosses fingers*
If not, I wouldn't budge. A discount should really be the least you get. In my opinion of course. Mulberry might disagree.
Sorry for the rant but rude people in retail drive me up the wall. If I managed to be nice to all customers at all times (not easy ... not easy ...) then so can anyone, especially when they work for a high-end brand.