She got spousal maintenance 20yrs ago.
You may as well tell OP she'll find a nice house for £60k if we're advising on what happened in 2005.
As PP says, mesher orders and spousal maintenance aren't a thing anymore. Yes, once in a blue moon. These aren't particularly exceptional circumstances. OP is going to be told to get a job. Not tomorrow, no. But by the time the divorce goes through? Yep. She'll get given a little house, which all four of them will have to live in. She'll live off a mix of child support, benefits and then whatever she does to support herself. Plus, spousal maintenance is deducted from UC pound for pound. She'd actually be quite silly even to apply for it.
Again, as PP says, if the divorce is her idea it's a pretty daft one. If it's not her idea, she needs to start working out what she's going to do to support herself because divorce is no longer the gravy train it was for women once upon a time. Courts want clean breaks and independence as priorities.
Frankly, I'd be shitting myself in OPs position. Unless she's far North/Scotland, £300k buys very little house. And this has to have three kids and an adult in it. Living in it Vs being squeezed into it will make a lot of difference to daily lives. Then she's got to run it. And support 3DC. He could quit his job tomorrow and go self employed, so she can't bank on living off child maintenance for the next decade, and even at the potential current amount, £2k a month is less than what someone on minimum wage earns. Again, never say never, but unless GP are on the doorstep to have the kids a lot, she's not got the option of getting out to start dating and meeting someone, and then they need to want to become a SD to three small children. Although the ex has offered 1 night a week, maybe that will help.
Circumstances as to how all this came about are fairly critical.