Overtime I have picked up a few 464's that have been faulty. I only get them because they normally come with loads of games which I collect. Anyway one of the machines looked in really good condition but when powered it just gave a green screen (using green monitor for testing). I swapped the 40009 chip and noticed sometimes in came up with a light border of vertical lines so I knew I was getting somewhere. Next job I exchanged 2 of the ram chips (I presume it's ram - 2 banks of 4), switched on a was presented with the normal display with all keys working ;D I have since loaded 2 games on it with no problems.
Cannot believe I got so lucky with this as I am quite naïve with this sort of thing - still finding my feet :D