Hi Fauve
I think they can probably deduct the money if it's a direct debit, as there is usually a 'variable direct debit mandate' that you sign for: i.e. you've probably already given permission for them to take differing amounts of money from your account.
However, when they say 'they have to take a lump sum from your account' that sounds as if they are asking for permission.
1)If this is the case, I'd be inclined to decline permission, on the basis that if you had to claim during the time involved, they might not have paid out.
2)If they have already deducted the money, via your direct debit, which they are probably legally entitled to do, I would then look for compensation (i.e. to get the money paid back into my account, as it isn't your responsibility that they in error deducted the money from someone else's account. )
Claiming retrospectively for insurance?
I personally would argue the point with them: saying that I would be more than happy to pay for cover from now on, but that I most certainly would not be happy to pay backwards for it, due to my point above (i.e. not being covered if you had a claim, potentially).
I think this depends how far you'd like to take this: it does take time sorting this sort of thing out, but you certainly deserve a letter of explanation, and personally, I would expect to be reimbursed, if this money is deducted.
It is possibly better to go to the top of the chain: write a letter to the manager of your branch, or ask to speak to them. You are more likely to have a satisfactory resolution doing this, and this has always been my experience.
You may well be successful in this. It just depends really how much time you are willing to invest, and as I have said above, it'll probably take time.
Post back to say how you get on
Nicola