And don't forget the Cornish Engine Houses elsewhere in the UK...
David Bick listed all the significant remains of Cornish Engine houses in Wales in an article in
Industrial Archaeology Review, vol.12, no.1, 1989. He listed all that had their bob walls intact or had comparably full remains; thus very ruinous engine houses such as Llanfyrnach Lead Mine, Pembrokeshire, were excluded (this one has further collapsed since the 1980s). He listed 39 throughout Wales. Of these, one (Pwll Colliery, Llanelli) was demolished shortly afterwards. Most, if not all of the ones he listed were, if not already protected, listed or scheduled after publication of the article and a number have been repaired, most due to the efforts of the Welsh Mines Preservation Trust. I have only found one that he missed: the pumping engine house at Gorsgoch Colliery, between Cross Hands and Gorslas, Carmarthenhsire, which has three walls standing full height although the bob wall has either collapsed or been demolished.
Cornish engine houses were never hugely numerous at Welsh metal mines (due to cost of transporting coal to them, and the cheapness of water power) but they were built in considerable numbers at Welsh collieries. Unfortunately most colliery Cornish engine houses in Wales have been demolished, some as a result of ongoing plant replacement at nineteenth century collieries that worked into the twentieth century, and others due to the 'wipe the slate clean' land reclaimation policy of the Welsh Development Agency and many Welsh local authorities in the 1960s, 70s and 80s.