OK - so the progress today:
Removed CPU, Antic, GTIA, PIA & Pokey and tested in my 800XL (equivalent ICs were already socketed) - They all appear to be working as in the computer booted up.
De-soldered my Freddie IC from working 130 XE and installed a 40 pin socket - now that computer is reporting 8 bad memory chips - great!
Put the Freddie from the 'dodgy' computer into working computer and no life.
Put known working Freddie into 'dodgy' computer and now get a black screen - secondary problem somewhere. Interestingly the Sys-Check isn't initialising with the this configuration.
Progress of sorts today although now have a lot of memory chips to de-solder from the PCB - got a load of 41645 arriving Tuesday all going well, hope I ordered enough for 2 computers! Off to order a couple of Freddie chips and keyboard membrane as well.