OK.
Got OS 4.1 Classic from AmigaKit (thank you) and the install was relatively painless with one exception - my Deneb.
I looked at some instructions here -
http://www.hd-zone.com/2011/06/optimizing-poseidon-for-amigaos-4-1-classic/ and they say installing Poseidon 4.4 is straightforward and I can accept the defaults and have it start automatically.
Trouble is during the install I elect to start it automatically whereupon the installer generates a Grim Reaper exception. Tried three times and each time the result was the same. Tried again by saying 'no - don't start by default' and the installer went through fine.
Rebooted the Amiga - all OK so far. Installed Trident and got that to come up and added the Deneb manually. Mouse was detected (USB wireless) and it all started to work. Rebooted again and got a Poseidon error message at bootup but my mouse was working still. Attempting to start the Trident GUI also throws an exception now so I can't actually configure it - but my mouse is working.
System is as follows:
4000 with a Mediator 4000Di w/Radeon 9250 from Elbox
CyberStorm PPC 233MHz / 128MB RAM
Realtek RTL8139 NIC (works in 3.9, sadly not supported in 4.1)
Deneb
Indivision AGA
16GB CF card for HDD
and that's all..quite a basic system in terms of addon cards (limited by space

)
Any help would be appreciated if you know what could be causing the problem. Can provide more specific error messages if required but the Miggie isn't with me as I'm writing this.
Thanks!