You make it sound like you only made one attempt to download the programme. Give it half a dozen tries - it's gone 9pm so tinternet traffic is past the peak (6pm)
If that fails, it's time to visit Windows Add/Remove programs
1. Uninstall BBC iPlayer
2. If you see Abobe AIR on the list, remove it. It ceased to be a component of iPlayer last year
I don't know what YouView is but it might conflict with iPlayer and you might want to temporarily uninstall that as well, to see if this helps.
Digital Rights Management (DRM) may be the thing blocking the download and iPlayer might be finding a Windows registry entry telling it you've already had your go at watching this programme. (speculation, on my part - don't meddle with the registry anyway!)
So, reinstall BBC iPlayer and retry the download. Hope this works for you.