I initially did load the Optimized BIOS Defaults. But I got so frustrated with it that I got a data recovery guy to mess with it. I had bad sectors on the boot HD which he fixed and I had him add another HDD. I now have 3 x 1TB drives. I'm pretty sure he 'mixed and matched' until he got them (i.e., the RAM modules) to work. He didn't change the frequency from the default (1333Mhz - I checked) and tried to keep everything as bog standard as possible.
Like I said, it boots fine with 24GB - when I remove the module in slot number 4, as directed by Derek, the data recovery guy. Put that module back and it takes 10 Minutes(!) to boot. I suppose I could number the modules and try swapping them around - if you think that would help.
Thanks.