Loan - in breach of contract

I'm hoping somebody can offer some advice regarding a loan.

I have a personal loan which allows me to make additional 'ad hoc' payments at any time without penalty. I have an ongoing dispute with the loan company, and would appreciate any advice anybody can give.

The dispute centres around the processes and back-end systems used by the loan company, which can't seem to cope with additional payments. On two occasions, I have made additional 'ad hoc' payments only to be rewarded by receiving missed payment letters, and payment demands. It seems if you make an additional payment close to your regular direct debit date, the system cannot cope.

In these circumstances, my original Direct Debit payment ends up in a 'suspense account', and it takes the Loan company around 10 days (and 3 or 4 phone calls) to 'find' the money. During this time, they consider to send increasingly threatening 'missed payment' letters to me, even though I have proved to them that the regular D/D payment has been taken (PLUS an additional payment). What is more concerning is that they have confirmed that it could have taken weeks to correct had I not prompted them. Very unfair when you bare in mind that interest is calculated daily on this loan!

Now - I think that the company is acting unfairly here. The loan agreement clearly states that 'additional payments can be made at ANY time..' - however, clearly their processes cannot support this. This is not an isolated incident, and the staff have confirmed that this issue will ALWAYS occur if you make an additional payment close to the D/D date (7-10 days). They are trying to tell me that this is perfectly acceptable industry practice - but I disagree. Before I escalate my complaint, I wanted to "sense check" my rationale - and hopefully somebody can help me do this!

My understanding of my rights/their obligations are as follows - but I'm no expert! Any help/comments/pointers would be very much appreciated.

1. Since the loan agreement states that additional payments can be made at ANY time, and I presume this document is contractual, is it fair to say that the company may be in breach of contract in these circumstances, since their systems cannot support their contractual obligations?
2. The company in question is a signature of the banking code
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   The banking code clearly states that members should run "...secure and reliable banking systems". Since the company could not account for my money for 10 days, I do not consider this secure/reliable. What's the consensus on this one - are they in breach? Would welcome any comments here.
3. I'm led to believe that BACS have a set of strict regulations which are issued to companies collecting payments by D/D - and I suspect that this practice may contravene them. Do we have any BACS/DD experts who might be able to offer advice?

Are there any other codes of practice which might be of use?

Thanks in advance, and apologies for the long post.

Anton Reeves.