ETI7 - does the 7 mean July ?.
There was an MK14 advert in the July 1978 ETI, see here
https://worldradiohistory.com/UK/Ele...1978-07-78.pdf
So it looks like that was the advert I replied to, and then bought the extra bits a few weeks later. Total cost for expanded MK14 plus cassette interface was £70.87 !
That issue of ETI has loads of good stuff. On page 11, as well as yet another SC/MP system, there is an article about fake chips, and another about rip-off copies.