I had an intersting reply to a query about a Regimental service number from the Lancashire Infantry Museum : Thank you for enquiry. At the moment I am unable to search the South Lancashire Regiment attestation entry records as they being digitally scanned as part a UK wide regimental museum project, and won't have access to them until, at least, the end of May of this year.
Durham Light Infantry ones were copied to Micro Fiche and not available online only viewable at the Durham Records Office after they were gifted to them by the (now closed) DLI museum. Some were destroyed see the update from Paul Reed here:- Invaluable Resource Tools ENLISTMENT AND DISCHARGE BOOKS WW1 registers. The attestation records for the Tank Corps and the Royal Artillery can be found on Findmypast. Similar thread here linking back to WW2talk here:- Attestation books - where they were sent by the MOD/Nat Archives Kyle