Sticking point before mediation - what would you do?

[AllInTentsWithPorpoises]

Slimy ex husband and mediation conundrum. What would you do if you were me? He's holding out on some last bits of paperwork and generally being a total dick before we can go to mediation. There is one account he is insisting does not exist but I know full well does and is a savings account. I actually have all the login details but obviously can't use them to check as it would both illegal and would send a passcode to him. It looks like I cannot force this as an issue as we are not (so far) going to court.
So, there is a missing bank statement for April last year, which i reckon has his bonus on it. All other statements start in April, this ones starts in may. I think I am going to ask for him to supply this instead and then agree to start mediation with the other bits to follow (they are not so important.) The bank statement I can ask for I reckon so I have a full tax year of salary to work out correct child maintenance. I'm basically trying to swap proof of a savings account for proof of a bonus to get pretty much correct maintenance and get things moving.
In general my solicitor has come up with a good settlement that I would be happy with if I get it based on what is in his form E, including a few points I can go in high with and use for negotiation.
I want to get this done. It's been going on for years. Decree nisi took 18 months from start to finish because of his dicking about. Finances discussions have been going on since January with form E's swapped nearly 3 months ago. I would chase the savings account but it could be very expensive to do and what if I am actually wrong and there's nothing in it? He literally fritters away £4k a month on random shit, I have the figures so this is entirely possible.
Does this sound like a reasonable plan?

Thanks if you've made it this far!

[Momentumneeded]

@AllInTentsWithPorpoises I'm not going to be much help to you but I'm in a similar position. I've requested several extra months of backdated statements (basically I think he's been stalling financial disclosure to hide funds being moved - in my case even more complex as he has accounts abroad). I am so frustrated with a system that lets them get away with delaying things. Like you say though, without being in a court framework, what they provide is voluntary so I'm not holding my breath.

Can I ask you about your prep for your mediation session? Have you basically sat down with your solicitor and his Form E and drawn up a wish list? How are you intending to approach it? I'm trying to get my head around it all. Should we both have a firm idea of settlement going in rather than it being something we work out in the sessions? Do you have a min threshold whereby you are prepared to walk away and go the court route? Are you capping the number of sessions? I'm really trying to avoid it but I'm fully expecting a v unrealistic wish list from him that will leave me impoverished and bully boy tactics. I am desperate for it all to be over.

[AllInTentsWithPorpoises]

Hi, my ex has accounts abroad too. And his previous salary and tax was too. It's a nightmare isn't it? I'm at the point where I think negotiating well in mediation is my best chance.

In terms of the paperwork, we swapped form E's and all the bits with it and I gave these to my solicitor who has come back with a suggested settlement based on what he has said he has, plus she agreed my list of further questions which asked for more stuff (like clarification on some payments that appear to be investments etc). Its the further questions he is being difficult about. Basically saying things don't exist when they do. I think I would like to do mediation and make it work but yes, I need to set myself a point where I would walk away and take it to court.

One of the reasons I left was him being financially secretive and controlling and a gaslighting, borderline emotionally abusive twat. I want this done but I need to be happy with what I walk away with.

[Momentumneeded]

@AllInTentsWithPorpoises sounding more similar by the minute. I think they believe their own lies. I've had some (really helpful) counselling and the upshot is that my stbx is a class A narcissist who constantly moves the goalposts to get the upper hand. It's all about winning, money and diminishing me any which way he can. Hence, not convinced mediation will work but I'm told the court route could take a further year (and I've no doubt he would put me through even more hell - he has a lot of cash to spend on fancy city lawyers) so like you I feel I have to give mediation a shot against my Sol advice to go to court and yes, effectively they are on a huge spending spree in the meantime to reduce the joint pot. Grrr. It seems the system is there to be flouted....