I've done a quick search it looks as though the LeakBot app is available on the Apple App store, so it may be that your iPad is a suitable option, as long as it's not too old to download it.
I suspect that the app wouldn't need to use a data allowance within the home; I'd expect it to use wi-fi or bluetooth, so you might not need a separate phone.
If you wanted a mobile phone, or needed an Android phone, you can get very basic ones fairly cheaply. The question would be whether you wanted to use it out and about as a computer (rather than phone or text). If you just wanted phone or text, then PAYG might be suitable. Otherwise, you can get a monthly tariff for perhaps £5 per month.