Geoff,
Thanks, got it now.
Back to the original question, 'should I try NiMH?', my money's on no. Overdischarge, while not a good idea with NiCds, is fatal on NiMH. Just a matter of time. Reverse charging (a very real prospect) will just hasten the end all the faster.
I suggest replacement (when it's needed, the original sets are still OK, right?) with 'what the factory used', NiCd. There are solid reasons.
Doug Owen