Mains/Battery smoke alarms

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
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Kalico wrote:

May be worth spending the extra 2 each and getting the ones that also have 9 volt batteries included. The SF ones seem to work OK and are easy to wire with some 3+E for the L N and signal.
--
Toby.

'One day son, all this will be finished'
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on price for single units but when buying 3 the difference is much bigger.
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