Update: I saw my pdoc yesterday.
He wasn't sure what to make of the depersonalisation experiences, and didn't seem too concerned (suicide ranks higher in his worry list). He said it could be a form of epileptic seizure or possibly anxiety.

I have been taken off all antidepressants (again) and put on a combination of Lithium Carbonate and Valproate. He said in my case the Lithium is useful for its effectiveness in reducing suicidal impulses, particularly when the is a high risk of impulsively acting on them (and I'm fairly impulsive at the best of times).
I'm hoping that it will work to improve my stability, as I am still rapid cycling a lot with about 2 troughs & 2 peaks per month.

The down side of Lithium for me is that it is likely to push up my weight and there is a risk of tremors which can interfere with my profession - photography. So this is a trial period for lithium.

The really good news at the moment is that since I dropped my Cipramil dose down to 10mg daily (tailing off before quitting it) I have had 4 consecutive days of being free of depression and wild agitation or hypomania, and have been feeling more alive too.