Debster, no practical experience but a few thoughts.
You will be entitled to time off for ante-natal appointments once you admit you're pregnant, so I think you probably do need to tell them before the scan at the end of May so that you can take that time. Lots of people don't tell anyone they're pregnant for ages so I don't think that's exceptional.
I think in your postion I would go for the brazen approach: get a meeting with your line manager, begin it with a huge smile and tell them you're very happy to announce that you're pregnant!! You could either be barefaced and say that you've only just found out or you could just let them think that by omission, but you should present it as a lovely happy piece of news - which of course, it is You don't have to answer any questions about whether you were trying, that is your business and no-one elses. They should know better than to ask!
If you are planning to return to work you could tell your employer that you are intending to return and that you will work up until week xxx of your pregnancy. They will know that they are not allowed to discriminate against you because you are pregnant and therefore shouldn't treat your news in any way other than with polite professionalism. Carry on as you were and prove your worth.
I WISH we lived in a world where it was accepted that women are entitled to work and have children and get pregnant, all at the same time. Where your kind of dilemma didn't have to be considered. For goodness sake, what does industry think would happen to the economy and society if women didn't have children? Grrrrrrrr
Good luck.