I have seen that the instructions are different for smoke vs heat alarms, and have installed a few systems. The heat ones must be ceiling mounted, the smoke ones can be wall mounted provided they are installed in the correct zone, per Aico's instruction sheets. It is awkward that there are rules about the hard wiring of these devices which makes damage to decor an inevitability, and yes, in Scotland there are many variations on how the lights are wired, but if installing to Part D then as it stands you have to hardwire them all. The method I have used is to hardwire the hall smoke alarm, one with 10 year Li battery too, and radio interlink module, and then use the non-mains heat and smoke alarms elsewhere as required, they also being the 10 year Li battery powered type with radio interlinked models. The manufacturers suggest that some Local Authorities may accept such a set-up as compliant with Part D, but I haven't checked locally. However, as you may know, the Scottish Government had a consultation recently about alarms in both the public and private sector, and amongst other things a particular section asked for views on changing the regs to allow long-life battery only systems, so that hardwiring and mains supply would no longer be necessary, given that technology has moved along considerably over the last few years. The responses I have seen so far have supported this change, as do I, and it seems likely that this "relaxation" will be confirmed when the results are published and the legislation changes completed. I think this is good news, as those who are put off installing alarms may well go ahead as it will be cheaper and less intrusive...not perhaps such good news for electricians, but maybe good news for safety as a whole. Just my thoughts...