can anyone explain how the matrimonial asset thing works.
say assets are £25k.
one party as debts of £5k.
do they each basically get £10k - or does the non debted party have to pay half the debt?
You dont give nearly enough information on the above for anyone to even get near to advising you on this matter.
If it was a straight 50/50 split then of course what you have said in your post makes perfect sense, but it all depends on so many different senarios.
Maybe if you post more information then you will get more of a truthful and fair answer.
In practice one asset/debt can be offset against another so in your BIL's case they might agree to pay off the debt and each take £10k, or alternatively he or she could keep the debt and take £15k with the other party taking £10k. It's a question of negotiation and what suits the individual circumstances.
Is his wife likely to need aliment (quaint Scottish legal term for maintenance) for a period of readjustment?
Whatever is agreed it's worth getting a solicitor to draw up a 'joint minute of agreement' which the courts can convert to be legally binding.