The bulletin board channel for STD C on Alphasat is on 1537.1000MHz. From that you will get the list of active channels and the Ground transmitting station each is associated with (loads of documentation available via Inmarsat website .....though they can't decide if they want to use hex or octal for station IDs, old or new region IDs etc... so be aware of that and that the decoders are also different...Tekmanoid uses new region names, scytale-c uses old ones)
The signals will likley be weaker than the Classic Aero you have already decoded.
Depending where abouts you are (I'm guessing UK) you might also be able to pick up Inmarsat 3F5 but for std-c off that your patches are very unlikley to give a good signal unless mounted on a dish (in which case remember polarity matters)
The best 'hobby' decoder is http://www.inmarsatdecoder.com/ but that is not free.
In terms of message types decoded the next best would be Tekmanoid (not free, not even a basic demo version available to test). http://tekmanoid.com/egc.shtml
scytale-c has other possible advantages if you don't need all the message types - it's easier to set it up to monitor all of your available std-C channels, it is (possibly) more actively in development. For most people the biggest advantage is that you aren't asked to pay the developer for his time and effort. Alex will certainly appreciate feedback on what he has done so far as he teaches himself about the math(s). https://bitbucket.org/scytalec/scytalec/downloads/
There are plenty of other decoders but they are generally aimed at a more professional market and thus expensive or else the std-c is a kind of half-hearted add in (multipsk springs to mind...great software, hideous gui, ok for EGCs)
For scytale-C to maintain a good lock it needs a good signal. The same is true of Tekmanoid though it is slightly more forgiving. Scytale-c autotune also works better using a larger bandwidth than it ought to need...but it will work well given those two things. You can easily set up both decoders to comapre them
Setup - for scytale-c you can use the sdrsharp plugin and the quick-ui - no need to download the standalone decoder unless you plan to use it with a different audio source (eg multiple virtual audio cables). For Tekmanoid you'd probably use a virtual cable as you have for Classic Aero. In sdrsharp Modulation USB, bandwidth around 2800Hz (for best autotuning, the signal isn't this wide), audio filter off. Tune to above frequency and start audio. Enable the plugin and you can fine tune by eye or check the auto-tuning box if you prefer to let the software do it. Fire up the scytalke-c quick-ui and ensure the UDP port matches the one that is set in the plugin (from memory 15003 was the default for both). Whatever settings you want for logging/messages/display and then click on the big blue arrow to start it running. You should see the packet number incrementing every 9 seconds and messages appear when they are available.
If you have watched videos of it being set up can you not pause the video repeatedly to give yourself more time (this asumes the videos do actually show you what's what instead of the usual goontube rubbish that serve as scrounging revenue generators rather than being detailed overview/setup of software).
No idea if groupsIO has a PM function but if it does feel free to ask questions via that method as this isn't really sdrsharp discussion.