Sale of freehold flat - Death of shared freeholder

I am looking to move house from my 'leasehold, share of freehold' flat. Unfortunately, the joint owner of the freehold (the owner of the other flat in the two-storey house) has very recently died. I was told by a local estate agent that this would make selling my flat more challenging as the freehold interest could not be transferred while the probate application process is ongoing on the flat above (please excuse me if I've got the terminology wrong!). I have found the online guidance slightly confusing and was wondering if anyone can clarify where I stand on this. Will I have to wait to sell and if so what kind of timeline might I be looking at?

Ellen - your terminology is spot on so no apology needed. It's your conveyancer you really need to rely on here as if the freehold is jointly owned then the surviving freeholder can deal with the legal title/ownership and transfer it.
However the death/probate application and registered information may still impact and make that difficult in the scenario you refer to I suspect the freehold title refers to the two of you and a form A restriction. The restriction is normally linked to the term 'tenants in common' and seeks to protect each beneficial share. It may not be an issue if transferring the freehold separately and for no monies (the sale/purchase monies will be in the leasehold transfer) but there maybe issues linked to the deceased's estate that mean for legal reasons you have to wait for probate and then act with the executor as appropriate on any transfer
I would strongly recommend speaking to your conveyancer for guidance