Right, just followed spiceyokooko's advice and started with a fresh install. No luck, same as before.

Have also followed ojfd's tip re getting the Presto extension to load first.

Tried junking the System 7 enabler as per OlePigeon and only having the Presto extension. Well, with the 32MB RAM installed on the motherboard, nothing (as usual) other than the chime. But with the RAM removed it will boot, albeit with the Presto extension bearing a red cross and then a message saying the accelerator did not start up correctly...

Bugger. Any thoughts?