Houston we have a problem!
Tried a last few adjustments this pm since posting like tidying up the wiring and changing the CPU to a 4800 for the extra speed and cache.
But - the CPU will not work if the lock down bar is put into the locked position so am going to try and just fit the cooler unit as the machine starts if I hold down the CPU with the bar depressed. Cannot see any bent or deformed pins and the original Sempron works ok and so did the 4600 but even it will not now unless I do the same as for the 4800. Seems like the lock down moves the pins just that tiny bit to break contact.
Problem solved a dud second hand CPU no way of telling until this I suppose - all well and machine off again