Coggys,

One of the first things I would do is check both this site and tripadviser.com for hotel reviews on the ones you have quoted. Quite often 'good deals' are for hotels that have bad reviews or that are stuck out on their own in the middle of nowhere - not always, but definitely worth checking first.
The prices sound pretty good.

TC

First a word of comfort hotel standards in Thailand are very good...better than Med for example. Its one of the few places I would contemplate a two star hotel.

I've stayed at the Nova Samui a couple of times but it was a good few years ago so my comments should be taken with reference to more recent reviews which you can find on web. Its a nice two/three star hotel, not on the beach but a three minute tuk-tuk ride away. Its also not suitable for those with mobility issues as there are an awful lot of steps. Its not luxury but pleasant enough with only a small pool which means you will have to spend most your sun bathing time at the beach.

The Orchidacea is near Kata Beach (great beach) on Phuket, set on a hill above it (used to be Pop cottage). Not been to it myself but know the area as have holidayed at Kata a few times.

Have been round the Krabi Heritage...in itself its a nice three star property but the pool had the feeling of being located in a car park and its not in the quietest of areas.

All these hotels are in reasonably lively areas with easy access to bars and restaurants.

Price wise seems OK...I would though maybe see about upgrading the Bangkok hotel..this usually only adds a few pound to the deal and its worth it to stay in the lap of luxury.