Gentlemen
Thanks for the tips. Opened the machine up today (with the help of a 1200-owning knowledgeable friend), and lo and behold someone had fitted a ROM switcher, but managed to put the ROM chips in the wrong way round (we believe), either way the ROM chips were completely fried (charred on the tops). Ah well, I've salvaged some spares...back to the small ads/car boot sales/eBay to find a working one...