A couple of things - (And I apologise for the long, rambly answer! )
By law, you donlt have to tell them until you are 25 weeks pregnant but you won't elegible for time off for appointments etc/risk assesments and what have you until you do inform them, in writing.
You won't get SMP from them whether you carry on working for them or not. You will have to apply for MA. And your entitlement to that will not change once you've worked for three months because it's calculated on your pay for any thirteen weeks in the 66 before the baby is due. And, if you do claim MA, you can claim it from the 29th week of your pregnancy, so if they do let you go, you'd not be without income too terribly long.
In terms of your employer keeping you on, though, I think you'd be far better off being honest about it reasonably soon. If you wait and it becomes obvious you're hiding something, it will poison your relations with your co-workers and managers. Remember, if they do keep you on then, by law, you'll be telling them you're pregnant two weeks after the end of your trial. There's very little chance that anyone will see that as anything other than the deliberate deception that it is.
I agree, I'd stay quiet until after my scans, especially with your history - that's perfectly reasonable - but much beyond that, I wouldn't. Letting everyone know where they stand is much better for you in the long run.
They can't 'fire' you for being pregnant, true, but there's an awful lot of people think that being pregnant makes far more of a difference than it actually does.
As a previous poster said, if you've been working for a company for less than a year, you have very little legal standing for an unfair dismissal case regardless, so waiting until after your three months won't help you. All they have to do is prove you weren't dismissed because of your pregnancy and that wouldn't be hard to do in the current climate at all. Most employment lawyers won't actually even discuss a case where the employee has been working less than a year because there has never been one successfully prosecuted. So no, not a guaranteed 'clear win' at all. Unfortunately. And not one that's applicable at all, if you haven't told them that you're pregnant. In some ways, keeping quiet makes it easier for them, because they can't dismiss you for what they don't know! Even if they find out by rumour - you'd have to prove that they did know!
It's an awkward position, I know, because I found out I was pregnant just after I'd agreed a job change, but it's more hassle for your employer to replace you for the rest of your three months than keep you on (think recruitment costs, lost time, more training etc) and I really do think you'll stand more chnace of keeping your job at the end of it if you're honest with them, than if you're not and they find out anyway. I wouldn't be keen on an employee who thought they could hide something like this from me to 'trick' me into keeping them on.
One to think about carefully, either way.