Apologies for slow reply-just lost a very long post so will try again. Thanks for your responses and believe me I'm taking everything on board.
The option to delay repayments was along the lines of what you suggested Yogii but I don't want to have it hanging over our heads and think we should just start paying it. I am equally reluctant to have his parents bail us out as it seems so juvenile, ridiculous and embarrassing (plus I do not want to be beholden to them regarding discussing other future financial decisions).
Re getting independent advice, I will do that. He is waiting on the paperwork to arrive from the agency and armed with that in black and white (and translated), I will make an appointment to make a more informed decision.
We are not based in the UK but I imagine the system is similar enough here. On the website for the agency, it says that spouses and children are not liable for the debt-the debtor alone is liable. I think it's not uncommon that people either don't/can't repay so they seemed surprised and very helpful when he contacted them to arrange payments especially since so much time had elapsed. Any credit check in the past (i.e. previous mortgage applications, loan applications) did not highlight this debt but they also did not know where he was so you're right, that has probably changed. I have a mortgage on a house which I bought before meeting home so it's in my name alone and realistically, I think it will be a long time before we are in a position to buy.
Polly despite everything, your post made me smile, his parents could not be further removed from head-in-the-sand, financially reckless people! They are lovely in lots of ways, but can be almost irritatingly practical and frugal when it comes to spending money. (The Psychologist in me wonders if that is why he is so awful with money-almost a reaction against them!) I was really surprised that they suggested it and it made me think it wasn't such an awful idea but I will of course look into it further.
Thanks for all your posts, I hope I don't sound like a trusting idiot by these posts. I am by nature quite sceptical and cynical but I think I can get caught up in the emotional aspects of his lying and not want to face the practical issues re the debt-this thread is forcing me to consider everything. In lots of ways, he is great and we are good together but this to me is huge...