Solicitor insists on visiting

I believe this is standard practice Galen. Solicitor has to see the document being signed/witnessed, and satisfy herself that you are not being coerced into signing. If you don't want her coming to the house, you should arrange to meet at her office.

It's to check that you are sure about the terms and are not being coerced in any way.
Is she a solicitor, or a legal assistant?
If she's a registered solicitor, it's all part of her professional responsibilities.

My partners father is leaving him money but nothing to either of his daughter's who cut him out of their lives when he left their mother. I hear something recently on radio about challenging wills and saying you should leave a written explanation with the will if there is an thing which could be challenged/seem unfair. Wasn't there a recent court case?

I think she has professional responsibilities to ensure that she is dealing with you and not being hoodwinked by someone else. I think, for the sake of ensuring your family get what you intend and don't fall out in a lengthy and costly Jarndyce -v- Jarndyce row that you either visit her or she visits you.