We are being asked because they do not wish my eldest to benefit from their money.
They acknowledge the money was a gift and they acknowledge that there is no legal basis for the request.
I'm sorry, OP. What utter nasty, nasty arsehold behaviour from them. 
I thin you might be wise to avoid any of this conversation with them yourself - anything you say will be translated by them into you being a grabby fucker and likely to entrench them into their position.
If I were in your shoes I'd be asking your H to tell them exactly what he thinks of their shitty request and the fact that this is in danger of damaging their relationship with their son and their grandchild(ren), let alone his wife and his stepchild. That £80k (or £160k) is of use to their son and his family right now, and their actions will force their son and their grandchild(ren) to downgrade their opportunities if they were to claw 160k out of you at this point.
They gave the money as a gift 15 years ago. They didn't jointly invest it in property with conditions about reclaiming the money and the interest when they made their GIFT, They didn't attach conditions to it about "oh but we want it back with interest if you marry somebody who already has a child that isn't yours!". They don't get to make demands on what happens to it now, and they certainly don't get to make demands that it excludes any individual, and expect that those demands won't have a cost to their relationship with their son.
A much more reasonable and proportionate thing for them to have done would have been to have had a carefully worded chat with their son about their intention for that £80k being to benefit him and any direct descendants he had, and expressing their hope that he would ensure that the portion of his estate that came from them would be passed to his own descendants upon his death without being diluted by being shared with any step children. They would still have ZERO rights to demand that he did as they wished, but I imagine it would have been better received than demanding £160k from him now with the explicit intention to ensure that his stepchild doesn't see any benefit from it, in terms of living in a house funded by it or of inheriting.
If I were your H and I also felt that I wanted to take legal steps to ensure that only my blood children benefited from either all of my estate or a proportion inherited from my parents, I'd have a chat with them reassuring them that this was either already or would be taken care of in my will, whilst also making it clear that their current demand was absolutely outrageous and extremely damaging to my relationship with them. Under no circumstances would I give £160k to them, and I'd only give £80k to them if this was the final straw and I was telling them to fuck right off with their open hatred towards my wife and stepchild. Please do show him my post if you think it would help him.