It is not wrong for Grandparents to see their granchildren even though their son / daughter is DF'd. Dig around long enough on the WTL and I'm sure you will find something, a mag(whatever) to back that up.(if your M-I-L cared that much, she would have done this).
To be hoest, calling Bethel? Well, I guess you could ask them your question re. "the rules on seeing granchildren etc". Other than that, sorry but it won't accomplish that much(esp in London).
Personally, I would do the groundwork and then confront the MIL face to face, the Bro at Bethel may quote scripture and ask you to do this (I know this will be tough), she will be shocked to hell, especially when you have WT refs. You may even be able to say "I called Bethel and....".
Whatever you do, don't let anyone convince you that you are to blame, this is not down to you. Obviously, I cant give any info, but I have a feeling that I may know the "ex min servant that was sent to prison for abusing kids" - bastard! (please excuse my french). Actually, I got hold of his mobile number and tried to call him to tell him (well, words can't describe) but is was dead. Hope I never see the guy, wouldn't be pretty !!!!!