I would definitely install a hard-wired alarm. They are cheaper and more easily customisable (ie. you're not tied down to a particular make and type of sensor should you wish to expand).
I hard-wired my own alarm on the garage at my previous house, it only took about three hours, including bell box, PIRs, mag switches etc. It's not difficult to follow the instructions.
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However, I also have a question: I've since moved house, and the place where I bought my previous alarm components (TSS in Exeter) has been bought out. It seems that no matter where I look, component prices seem to have at least doubled in the last 5 years. Can anybody recommend a website or mail order company that sells components at a reasonable price? As an example, I installed my previous alarm at a cost of £130 (bell box, control panel, backup battery, keypad, DualTek PIR, mag switches, cabling). To do the same job today would cost me over £300!
Thanks
Jim