I then moved on to the TTL chips surrounding the cpu, I found a 74LS245 @C12 with two output pins (11 and 13) stuck low and all the inputs were pulsing away nicely. I cut these two pins and checked them with a logic probe at the board and the lines were now floating instead of low, this confirmed that the 245 was bad and that it wasn't something else on the lines forcing them low.
Socket and new 74LS245 fitted.
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