How do I find out how long a UK train should be, in advance online?

On the Great Northern line between London's Kings Cross, Stevenage and Cambridge, services can be operated by 4, 8 or 12 coach trains. On parts of the route, you can also get 3, 6 or 9 coach ones. When you get to the station, the departures board will show things like "15:04 to Kings Cross, Great Northern Service formed of 8 coaches" or "17:22 service to Cambridge, Great Northern Service formed of 12 coaches". Other lines / bits of the country often have similar things.

I know that in the late afternoon, some of the services heading south on this route are 4 coaches, and you almost always have to stand on them. Some of them are 8 coaches, and you can normally get a seat, at least as far as Stevenage. I've tried complaining about the overcrowding on the 4 coach trains before, but due to the wonders of the "competitive" privatised rail system, the response was unhelpful and overcrowding remains...

What I'd like to be able to do is look up which services will be 4 coach, and which 8, so I can plan to get one with a chance of seats. Because of the service interval, I don't really want to get to the station to read the display board to see which that'll be, I'd like instead to check online.

I've had a play with the National Rail Enquiries Online Journey Planner, but while that's happy to tell me who operates the trains, and if they have 1st class or bike spaces, I can't seem to get it to tell me if it'll be a 4, 8 or 12 coach train.

Clearly this information is available somewhere electronically, otherwise the display boards in the station wouldn't know to announce the train lengths! How can I go about looking it up myself?