Is there any way I would be called as a witness in Ex's legal proceedings?

[Tearsoffrustration]

The father of my DCs (we have 50/50) has started processings to gain access to his unborn child (with someone else) has anyone got any experience of this?

Family court is a bit different. Ime, (but I could be wrong here) his solicitor would need to request that you appear as a witness, and the judge would have to agree. But my solicitor once said she'd never request a witness who wasn't 100% willing to talk in her client's favour. There would be no point because of the risk involved.
So although possible, I would say it would be very, very unlikely. A better scenario for him, provided you get on, would be to ask you for a written reference that he could submit.

How's he going to access an unborn child?! Sounds like a waste of money to me - presumably the mother is denying it's his and I can't see how he can prove it until the baby has been born and they can run DNA tests. At best he's only going to get very small periods of access when the baby is first born, building up over a long time, because the court is going to recognise the baby will need to be with its mother a lot, particularly if she breast feeds.

Also agree with pp - you're unlikely to be called in family court. From my own experience they're not too interested in actually getting to the truth of anything put before them. They don't test evidence in anything like the same way a (you'd hope) criminal court would.

Mess. Avoid avoid avoid.