Having contacted our landlady she advised us the plumber would be arriving yesterday morning and he duly turned up. He got the tap out readily enough and then disappeared of a couple of hours until we got another email from the condo owner.
She advise that her man was unable to purchase a new item (I was fully aware, having spent half a day looking for one myself) and in the end he had to get in touch with the manufacturers to get a new one sent out. In the meantime he would re-fit the original one and bodge it so that we would at least have hot running water.
When he got back he did as expected but the mechanical sink plug seemed to have stumped him as it was a live version of Meccano. It must have taken him well over an hour to "fix" and he couldn't wait to get out of here.
The upshot is that we now do have hot water and a plug hole stopper that operated a few millimetres from permanently closed and we can expect a new tap assembly at some point in the future.
Not complaining, very happy at the landlady's instant response to our problem but as ever, thing never seem to go smoothly when there is water involved.