Sorry, @Itisallgoingtobeok this thread has become too long for me to check all through it. So I just hope I'm not wasting your time!
Have you had your own solicitor check all through your partner's will, or proposed will, as although what your partner's lawyer is proposing is on the face of it normal, in fact, good practice, I do not like the part where his half of the home becomes theirs on his death, and that you have to run any changes you may want to make, by them first.
The legalities should at the very least, say that until your death, or you moving from that property, you have full responsibility for the upkeep of the property, but any changes you may wish to make to it, eg adding an extension, or changing the loft from being an attic, into being another proper room, with the necessary staircase, and, of course, planning permission if needed. However, there would also need to be a section, or a codicil if already written, that stipulates that an agreed third person, perhaps another, but independant lawyer, has to agree that any changes won't reduce the value of the property as it already stands. Also, if any changes increase the value of the property, the beneficiaries of your partner's will, will in fact still receive half of whatever the property's final value when sold, is.
It might not seem fair to you OP, that any of your own money invested in the property should then be shared with your partner's beneficiaries, but that would be the expected outcome for the beneficiaries if they are not to have control of their half of the property as soon as, in this case, their father dies. This is because your partner always wanted his children to have half of his home's worth when he died, but if he agreed to your terms for the interem dates between his death and yours, he would, very fairly, feel the need to take account of the fact that they may have decided to not let you make any changes you may want to make to the home, if you were the only one who was going to benefit from those assumed improvements.
I certainly think that in respect of the above, your half of the property should not automatically be willed to your partner, if you were to die before him, but as with you, he should just have the right to live in it until he dies. So, I think that after you have both had meetings with your own solicitors, the two of you need to have an honest, and in depth discussion between the two of you.