CISL will most probably have sent your letter to Halifax as you've asked for documents and not done as they want and paid up without question. I've had letters from CISL before and they just do debt collection just like all the others. I find that as soon as I respond and ask for information, they stop writing.(posted 7865 days ago)It'll probably be best if you can deal with Halifax directly as they will have all the information, but I'd hang on until they can be bothered to write to you directly rather than sending in a lackey. If you get more letters, then just write back asking for the information you want.
CISL will just have the information which Halifax have provided them and their own notes and unless you can afford to, I wouldn't bother with a SARN, although it might be interesting to know what Halifax are saying about you.
Even if Halifax are good at their compliance, having all the information they have about you in your own possession isn't a bad idea.