Here are some suggestion which I hope help you:
1. contact the RHF again and ask if they have a military historian. Contact him/her (if it's now a TA regiment, no duty personnel will have time to help you and probably aren't qualified).
2. Find a military history organisation near you, although try to join a senior association, even if it's further away,eg London or edinburgh.
3. A really good one used to be the Naval and Military Club, as it included ex-military and interested civilian members, it is also an "old boy's" club with networking value.
4.Try the Imperial War Museum, ask if they have records or ask for someone who can advise you.
5. Australia has the National Archives. This is where all such records are kept HERE. For example, I can see my WW1 uncle's service record online for free, but I also know there are sealed papers at the NAA containing hs Court Marshal records (got drunk and punched out a French adjutant). To open the sealed papers is a cost, but at least I know I can do it.
6.where did your grandpa serve? You might get information about his unit from those places inc France or Italy.
7. Who has the Regimental History? Where is this kept?
8. Your grandpa would have appeared many times in the Unit history because he was the WO1, or Regimental Sergeant Major (RSM).
RSMs are professional soldiers par excellence and every Battalion (about 500-600 men) had only ONE RSM. The RSM is the most senior non-commissioned officer in the British (or Australian) army and if they have a motto it is something like "don't muck wi' me".
9. Ask your local university how to research him and who to contact. You don't have to be enrolled as a student.
10. It is very interesting that your da's old record is marked classified near the war's end and that IS worth investigating as it may mean a special task.
11. If you're neat and presentable and your hair's not too long (and do NOT wear anything pseudo military) try and get invited for a drink at the Mess of the regiment. Try to get invited to the officer's mess first, but later ask to see the Sergeant's Mess, because that is where your grandad would have been (in fact, he'd most likely have been Mess President). This will give you a physically memory and connection with your grandad's regiment. be polite and interested, because regimental help can open doors or close doors to you.
(Take a uniform photo of yr grandad and his service record, but have a sealable plastic container to protect it from beer!!)
Cheers mate,
T