Hi Steph,
The next letter you need to send is a Letter Before Action - not sure whether this is what it is called on the Martin Lewis site as I haven't even seen those letters - but if it isn't then I think you should use the template from this site (Letter Before Action). It won't cause any problems to and ideally you should give them 14 days before you begin action - it shows the court you're giving them a chance to settle.
I don't want to seem like I'm being really negative but there's no point in referring to any claims against other banks previously as it becomes irrelevant when you're dealing with a different bank, and they won't pay any attention anyway. It certainly won't make any difference to your claim.
Just proceed as normal and you will get there. Good luck.
Lucid
