Statistically speaking, Mesher Orders are an extremely rare outcome of a case that goes to court. I think a study done in 2015 showed they occurred in something like 1% of divorce cases with a financial settlement and most of those cases were short term, normally for teenagers to finish their education. That doesn't mean they are never awarded, or never awarded in the long term. @Flowertight must have a case where there is a reason she is incapable of having a mortgage solely in her name that is sufficiently large to meet the needs of her children but this scenario is quite rare in itself these days. Or she agreed in negotiation.
However, there are an awful lot of factors to think about and one of them is indeed how to pay the mortgage in a Mesher Order scenario. Spousal maintenance is rarely an option nowadays. Such is the cost of living that the non-resident spouse doesn't tend to earn enough to be able to afford spousal maintenance nowadays unless they earn well over six figures. Again, that's not to say it doesn't happen all the time by negotiation, but in a court only around 16% of cases have a SM element and most of those are nominal (i.e. about 1 pence a year). However, even where SM is payable, often the recipient is eligible for universal credit and therefore being paid this money for the mortgage reduces their UC claim £ for £.
Consider this scenario. With a one year old the OP won't be expected to work but say this is in 3-4 years time. The OP will be expected to work part time and earn at least minimum wage, perhaps bringing in £1k a month. Let's say the ex is on £100k, so paying about £1.3k a month in CM for the foreseeable future. The OP can also claim about £700 a month in UC and about £200 in child benefit, so a total income of £3.2k. The OP's ex meanwhile will have an income of about £4.2k after paying CM. A court could award some SM but the first £700 would just wipe out the UC claim. By that point the OP would still be on £3.2k and the ex would be on £3.5k. However, the ex is likely to have much higher housing costs that keep rising, because they are unlikely after paying CM to have enough mortgage capacity to be both tied to the FMH and to buy another property so further SM is extremely unlikely. In other words, SM isn't really an option to pay the mortgage.
What's actually more realistic these days is a charge back. This is where the OP would have to get her own mortgage and pay it in full but the ex might have to wait for his capital with a charge on the property until a trigger point. She will be expected to downsize - within reason - to achieve this but will most likely get most of the equity from the FMH. She will also be expected to maximise her earnings to do this, although no one is going to expect someone with a 1 year old to work right away.
It's never really possible to predict what a judge is going to do but I would estimate a Mesher Order for 5 years with a 70/30 split could be a likely outcome in a court.