The new filters could be construed as "unsolicited goods"? ..so get them to come and collect them ...but charge them for your inconvenience for the time it takes them. I would have thought they'd just have said"sorry, it was a mistake, have them on us". They must not have many other customers if they can spend the time to take the old ones out. I wonder if the new ones really are new? maybe they did't change them at all and are frightened of you having a look and finding the old ones.
If it were me I'd want them to put either refund for the leak repair or put it right for free or a discounted rate and I'd tell them to sod off about the microfilters but still demand a refund, taking legal action if necessary.
I'm taking my M3 into Cooper Cobham at the weekend for a "technician" to take a look at. I'm dreading it.