Again Martin's site is the source of the VDU - here is a link to one of the articles with his repro Issue II - be careful to read as he ended up doing an Issue V but the top side picture may be useful.
https://www.8bity.cz/replika-mk14/ja...sinclair-mk14/
There is an English flag on the right had side which is pretty good translation.
The cassette interface page is harder to find as it is not on the index:
https://www.8bity.cz/2018/science-of...rface-replica/
The documents above are from 2015 in his blog that cover the early work on main board, VDU, cassette and programmer:
https://www.8bity.cz/2015/sinclair-s...om-programmer/