If I've found births on familysearch.org with the same indexing project number and source film number does that mean they are definitely related?
How can I then view that source to see if there is any more information?
It doesn't definitely mean that they were related, because theoretically there could be 2 couples with the same names christening children in the same place at the same time. However, it's usually a good guide, and you can use the censuses to check out whether there was only one couple named John and Eleanor ROBERTS in the area.
To view the source, you would have to look at the original parish register, which (in this case) has been microfilmed: Shrewsbury Archives should hold a copy, or you can order it (for a fee) to view at your local LDS centre. If you're very lucky, a kind RootsChatter may be able to look it up for you; however, it is unlikely to show much more information than the transcript.
I can't find a death record for William Roberts b 1.2.1821 and bapised on the same day but he doesn't appear on the next census in 1841.
Remember that on the 1841 census, ages of adults over 15 were rounded down to the nearest 5 years: 19 year-old William should therefore be recorded as aged 15.
