I have all the forms printed and ready to go and my wife has said she'll agree as long as I pay all fees. I'm happy to pay to get away from her but I'm scared she'll get the papers and go back on her word and I'll lose my £550, I've had to save hard for that money and she knows I don't have any more.
As your wife said that she is agree you have to trust her is the best option is to get the lawyer involve in all the proceedings who will handle all the legal work and your wife will also not change on time as it would be legal procedure.
For the ending of the marriage, she's unlikely to be able to cost you any more by being vindictive or anything. The financial resolution, however, she could easily blow up to monstrous expenses just accidentally. My advice is to correspond by email or text and print those out with dates and times. If she's negotiating a settlement, then it'll be easier to prove that the amount she asks for is the amount she's getting.