Wow the fact that the main concentration of this new thread is about the fair has left my head spinning as hadn't really considered it in too much detail other than as a poor excuse...now my brain is on overdrive trying to figure it out!
My thoughts are going round in circles:
1) It was an attempt to divert the search away from the house - but since there was time to come up with a story then if this was the case why did SH not just say that TS had said she was going to the fair instead of shopping?
2) They were disposing of something in the area, evidence/drugs but then if you're preparing for a police search then why be so concerned about that when there's a body in your loft?
3) Maybe CS/CB allowed TS to get away with more than NS did, maybe she allowed her to go off to meet friends or stay out overnight. SH knowing this could have used it an an excuse for her absence and when she didn't come home they went to an address or area in the vicinity of the fair in case she was there (would have had to be someone dodgy not to come forward). When she wasn't SH convinced her to lie or she'd have to admit to allowing such things in the past, if they knew there were CCTV on route they'd need a reason why they went in that direction.
4) TS was used to transport goods by the family, again SH could have told CS/CB this is what she was doing and when she didn't return they went to the address to look for her, again assuming in the vicinity of the fair. Good reason to lie about why they went in that direction.
All this is assuming that SH had already committed the act by the time CS/CB had returned and that she had no knowledge of the fact. In fact I know they're pretty far fetched assumptions but I can't work out (thanks to you all!!!) why they felt the need to add the fair into the story or why not simply say that's were she said she was going to meet friends....