If he is self employed then his bank statements may make interesting reading. I would be very surprised if someone who could command an 80k a year salary relatively recently is now only "worth" £5k a year.
Does he have a limited company? If he does, then you would need to ask for company bank statements etc as well as he may be "hiding" cash in there.
If he is keeping his declared income deliberately low but building assets elsewhere then a: these will be part of the marital pot and b: If CM is likely to be low as a consequence, a judge may well be inclined to suggest you have more of the cash pot.