mmfc wrote:Any checks I can make to confirm that? Just it will be a bit of an pain if I have to get a new one as obviously no one is making the B anymore... And I think it is just about to reach a year old, which will mean it is out of warranty too.
"Any checks I can make to confirm that?" 'That' being my suggestion that the SD card reader is suspect? Beg, borrow or steal (the last option is not recommended) 5 minutes of access to another RPi and try booting it from one of the SD cards you say should work.
If the SD card slot is faulty it is possible (but definitely not trivial) to purchase a replacement card slot and replace it. (But obviously that would void any remaining warranty.)
If you conclude it is 'dead', why do you need to replace it with another Model B? Isn't a Model B+ a better purchase? But it isn't "obvious" that noone is making the B any more - the Raspberry Pi Foundation have given assurances that is not the case.