Your exact maternity pay depends on two factors:
- Whether you have been employed long enough to qualify for Maternity Pay (if not you get Maternity Allowance from the government).
- Whether your employer just offers Statutory Maternity Pay or any extras. They legally cannot pay you less than SMP.
You qualify for SMP if you have worked for the employer for 26 weeks or longer when you are 25 weeks pregnant, and tell them you are pregnant on or before 25 weeks pregnant (so after the 12 week scan would be totally fine). So unless they only put you on a proper employee contract 10 weeks ago you will legally qualify. Your employer legally cannot get out of it no matter how much they want to.
Whether the SMP is paid by your employer or the government is dependent on their exact circumstances and employers can claim some of the SMP back. However that is not your problem, it's up to the company accountant to work out what they are entitled to and how to claim it.
By the sound of it your employer won't offer extra benefits for pregnant employees but worth checking out your employee handbook just in case. That should also set out the rules as to who you'll need to tell and how (for example does it need to be in writing or can you just tell your manager in person?)
As for sick pay, you qualify if you had 4 days in a row off sick (these can include non-working days, so sick on Friday, weekend off, sick on Monday counts). However it's not paid for the first 3 days of sickness within an 8 week period and it's only paid at £87.55 per week. Your employer has to legally pay this. Again they might get government help but this is up to them to sort out, not you. From your post, it looks like you should get 6.5 days, minus first 3 unpaid, so 3.5 days, which work out at £61.28. That does depend on when the 1 day you went back fell though.
Again check your employee handbook to see if you get extra sick pay allowance.
On a related note, once you have told your employer you are pregnant they HAVE to give you paid time off to attend antenatal appointments, and they cannot treat pregnancy related sickness as normal sickness - so for example you can't be sacked for poor attendance if you are off with pregnancy complications (whereas you could be if you just kept getting lots of colds and weren't pregnant).
Your holiday should accrue as normal, for the purpose of employee benefits women on maternity leave are considered to be at work. So you need to have used your holiday up by the end of March and you'll get a new load of holiday on April 1 2015. So if you are due on 12th Feb, perhaps look at using up any spare holiday in mid-January and then switching to maternity leave at the start of Feb.
This is a good basic guide to your legal rights www.gov.uk/maternity-pay-leave/overview
Unfortunately I have very little sympathy for companies who are dicks about people being pregnant (or taking paternity leave etc). Who is going to be their customers in 20 years time? Who is going to fill the entry level jobs in 20 years time? Who is going to fill the senior jobs in 40 years time? Your kids! If no one took time off to have babies then every single company would go out of business eventually. It's the price they pay for long term profits.
It can be hard to stand up to your employer but don't let them swindle you out of your legal rights. Pregnant women are pretty much bullet-proof in terms of employment rights so any employment tribunal would find in your favour if your employer starts messing about.