I received my package from the UK today, containing a dual ROM, new CPU and two new multiplexers!
First up, I slotted the new ROM chip and connected the machine up to my Indy, which gave me the following:
This indicates that all the lower RAM is bad! (And I've just replaced all the chips with brand new ones, so this is highly unlikely to be the true cause. It must be somewhere in the RAM interface or the CPU itself!)
I sent RETROLIUM a message, asking advice as to which component more than likely has failed and will need to be replaced and I'll take it from there. (I really think that it's unlikely that both multiplexers have failed, so in my mind it's more than likely a faulty CPU!)
At least the ULA seems to be ok, as I'm getting an image! (It, along with the "dead cockroach", really give this board character!)