[Marsh.]

I think it's Mandy.

She was in the show back in the early 90s and on her return I remember she had a bit of a thing with Jack, maybe she was having a rummage through his stuff, found the documents and she remembered the fire from when she was on the square and is now using it to her advantage.

I don't think it's Heather because she has the enveloped planted on her at Christmas.

But I hate how they make the whodunits over the top ridiculous. Like all Archie suspects wore black gloves etc, now most of the square have been in Jack's bedroom at one time or another to make it harder to guess. lol