Finally received my replacement 6502 from Ebay vendor.
Popped it in and it was definitely the problem as everything appears to be working normally again.
Meanwhile, I've received the Cassette interface and RIOT gadget kit completing a full complement of the expansions offered.
I think I'll start with the Motherboard expansion and then the cassette interface. I purchased a real cassette recorder and some tapes from Amazon - it'll be fun to get that working.
Regards,
Allen