I have a few points, not to challenge what you've posted, but just perspectives:
To be honest, I've now found I've been penalised financially as some errors have been put down as a staff error (me) but it was my line managers fault for not chasing certain things up whilst I was on mat leave. Is this not illegal? To be financially penalised for something I couldn't change whilst on mat leave?
It isn't illegal to withhold a payment while investigations are going on. They should be clear with you however that if they subsequently determine you are in the clear, that you will be paid the amount owing, according to your contract of employment.
Please don't place any blame on your manager, it will come across badly like you're trying to deflect. They will see you as someone who isn't prepared to be accountable for their actions.
I've got the investigation meeting on Thur and I'm going to tear them to pieces. Is this the best way to go?!
No, it is not. It would be far better to attend the meeting without any predetermined goal to be belligerent or aggressive. You will gain more ground if you remain quietly confident, professional, and willing to listen to their perspective. You should acknowledge mistakes were made (a 5% error rating is likely to be viewed harshly). Remind them you were in extenuating circumstances, re your late stage pregnancy, and that under normal conditions your work record has been strong. Clarify it isn't an "excuse", it's an explanation.
I wouldn't throw any colleagues under a bus by saying "well, look at all these other people's work, they make mistakes too...". That won't come across well.
It's a very stressful time, but hold your nerve, stay factual and if you want to continue working there, think about how you can maintain the working relationship while still standing your ground.