What are people's recommendations for main and/or battery smoke alarms?

I want them interconnectable, but the only ones I already have (Dicon,
battery operated) interconnect using 2 core cable and it would seem that
some mains powered interconnectable may use only one wire as a 'signal'
line.

Anyone know? The ones I am looking at are ScrewFix http://tinyurl.com/xwxf

Also, I saw someone post recently where to put Ionisation and where to put
Optical but cannot find the thread.

That said, SF's Opto and Ion detecters are different brands so may not work
together. Any experience anyone?

Thanks
Rob