Dallas had huge curiosity value - hence the 3 million plus ratings for the start. But a lot of people I know said there was too much bland kids and not enough of the original cast and walked after week 1/2.
I disagree and have loved it, but admit to having a bit of a vested interest as I visited the set during the original show and was driven to South Fork in an Aston Martin.
1.2 is an OK audience for a Five drama and it has been around there for 3 weeks now. But I am sure they hoped for more given the hype. It fizzled quickly and the Thursday night repeat was quickly axed (replaced by old repeats eg of the Jack the Ripper documentaries) and moved to a Sunday morning slot.
Not sure Five was quite the right channel for Dallas. It might have done well on BBC HD and multichannel in the Saturday evening Killing/Wallender/Borgen slot. I think its natural audience were those that knew the show on the beeb 30 years ago and who often find Five a bit young for them.
Be interesting to see if Surprise, Surprise proves more resilient or follows the same decline after the initial curiosity audience switches off.
Though (for the last time, honest

) I would still like the BBC to use Dallas as a template to bring back The Onedin Line as a next generation plus surviving oldies continuation. I think it would work.