Don't know anything about replacing boilers but you can buy carbon monoxide detectors from all good DIY stores. Either electric plug devices or little sticker things that change colour.
WHS plus get your existing boiler serviced NOW. Shouldn`t cost more than fifty quid.
Apparently back boilers are inefficient
Not necessarily. We've come a long way from the 'Glow Worm Majorca'. My mum & dad had their gas fire & back bolier replaced a couple of years ago, and the new system is excellent. Can`t remember exactly what they paid the plumber (in the region of £3k to £5k), but the local British Gas gits quoted £10k, so shop around.