Holding the cartridge so that "C43" and "C42" are not upside down, each of OP1/2/3 has jumpers in the top position. This would appear to indicate the "256" position.
Thanks for the reply - weirdly though i had 2 showing 128 and one at 256 - changed them all to 256 and got a rom error, changed them all back to 128 and it works fine...
still not sure why this should be but i guess it was worth messing with!