Hi Georgiemacminx - as an older Mum, I opted to have the combined nuchal scan and Addmark blood screening tests done privately two weeks ago, and I personally think it was well worth it, as it's a far safer option than going for an invasive test. The detection rate for the Nuchal is 71% and the Addmark is 89% so we felt it was sensible to go for both. My results showed very low risk - 1:50,000 which was reassuring.
I've only had NHS scans prior to 9 weeks, so no idea if the private scan was more detailed than what you get on the NHS at 12 weeks. The Sonograpaher talked us through the scan, covering every aspect of our baby's development from top of head to tiny toes! We chose to have a DVD made of the scan, which I have watched daily ever since!
They also check and listen to heartbeat and a couple of other things that just help to ascertain that all is going well and your baby is developing normally.
As well as measuring the Nuchal fold, they also look at the development of the baby's nose, as an underdeveloped 'flat' nose can also be indicative of Downs.
Surely at 24 you're low risk, but if you're worried I'd opt for either the Nuchal or Addmark test, and if the results indicate you are higher risk, then perhaps opt for further testing?
Good luck with whatever you decide.
Minnie