I watched it last night too. Thought it was beautifully shot, stylish but not afraid to show 'grubby' parts of London.
Excellent acting apart from Alex; I can understand why he'd fall for Danny, who's a live wire, endearing, and heartbreakingly eager to be in love, but I don't really get how Danny would fall in love with him. I thought he was a charisma/interest vacuum.
No, I don't think Danny knew Alex was a spy. He told the police in all innocence that he was an investment banker.
He knew to look in the laptop because when he suggested going away for the weekend, Alex said very meaningfully, 'I need to change my laptop battery before I go – it's important' or words to that effect.
I think he knew to swallow it because he's watched spy dramas, just like the rest of us! Also, he has a slightly dodgy or at least very druggy past and has quite possibly smuggled drugs around and into clubs etc. I'd imagine he's no stranger to being furtive; but I don't think he has a background in spying.
Apparently Ofcom are investigating the show because there have been thirteen complaints about the 'explicit' 
'gay' sex. People really need to get lives. The sex wasn't particularly explicit and was in a context of love and compassion. I thought it was believable, very touching and not at all titillating.