Hi Garry, thank you for contacting JA, my name is ***** ***** I will assist with your two queries today.
1. In the majority of cases you do not require planning permission for adding earth and turf, the exceptions are where you have specific restrictions such as a listed building, shared garden, specific covenants in your property deeds, but these are very rare, and owners would normally have been made aware by their solicitor at the time. Since what your doing is not going to cast a shadow on surrounding areas, or pose a health and safety risk to members of the public or changing the use of the space you do not require planning permission.
2. You wouldnt need planning permission or building control as the wall is less than 1M high (but if it was over 1M it would only be applicable if it was going to be next a footpath \ road), your not changing the property boundary, or building a wall higher than existing fences. However you may wish to check this page here, to make sure it meets expected standards in terms of the thickness etc.
Please let me know if you need anything clarified.
Technically raising your garden platform above 300mm (using earth, decking or other structure) would need planning permission.
Since you looking at 900mm you would need planning permission, however not for the wall, but for the additional earth used in backfilling the wall, by the sound of it, from the front of the garden your already have clear viability over the rear garden fence (as your up the hill), the only likely objection I can foresee would be regarding water run off, but thats just a case of having a soak away.
Alot of this is common sense, every council is different in most cases they will write back and advise you do not need need planning as the ground your 'levelling' makes up 5% of the total space, however its worth talking to your neighbour first as its your neighbour who will raise concerns to the planning authority, and push them do do something.
In summary you do need to notify planning, but its within the Council's discretion to sign off on what your doing without a formal planning application.