So a very quick update -
I have now ordered the P3 Celeron 600Mhz, should be here in a week or so.
Carried out some further research regards upgrading the CPU;
It seems the main concerns when installing a P3 into the 600E are an irratating BIOS error requiring some F1 ESC manouver to bypass, and the onboard 32Megs of RAM not liking the increased 100Mhz BUS speeds.
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There appears to be an easy work arround that I have tested pre installation.
To make these changes you need to get into the Easy Setup Menus and select CONFIG - once there press ctrl-d to open up a handy and previously unbeknown to me HEX Editor (NICE

!)
Ok on the first section of code that appears you need to make a couple of changes;
Byte 20 - change to 0A from 02 to disable the L2 Cache (gets rid of the 127 error)
Byte 2B - change to 81 from 80 to disable the onboard 32Mb
Hit F2 and turn the laptop off (if you just reset the changes aren't applied)
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After making these changes I checked the system status with CPUz
DIMM Slot #1 reports empty and L2 cache is not present.
Hmmm but once I've installed the new CPU (when it arrives) I don't want to be cripplied with no L2 cache...
No fear work around #3 also tested and fine
I downloaded the Powerleap Control Panel here;
http://hnpy.onlinedown.net/down/PLeapCCP_V5_1_0_0_1.rar
You can set this little utility to autostart upon XP boot and force the L2 cache
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Well thats it for the theory - as soon as the CPU turns up I'll let you know how it goes in practise.