The 303's can be a nightmare to keep brass in one piece, due to as pointed out above, the generous sized chambers. I have had extensive practice at retrieving the forward portion of the cases when we wee loading them years ago.
Our solution was to only neck size them after the initial firing of a new case. Now some 30 years later I have found that most use a small rubber o-ring around the base of the case just ahead of the rim in order to keep it securely pressed against the bolt face. This allows the case to expand and fill the chamber to the exact dimensions. Then they simply neck size and their cases last quite a bit longer.
Here are some post which will help you out and ease the mind numbing issue of splitting cases.
This one gives great detail on the use of the o-ring trick.
Reloading for the .303 Enfield
This one is from several discussions on the Lee Enfield and techniques of loading for it,
SMLE Discussions
This one is from EnfieldRifles.com and is load with load discussions,
Reloading the 303 British
There are also several more great pages dedicated to handloading and shooting these fine old rifles. Do a few searches and you will find them pretty quick.
Hope this helps.