Much easier to work through an agent.
I don't remember CPH application mentioning about field nmbrs, I think it was about address etc, I presume you have a CPH.
You need entitlements, we had to buy some. Again best through an agent.
I think you need an SBI number (single business Identifer), but maybe that was to do with the tree planting someone did on our land.
Yes, I found out very recently, subdividing our fields made things more complicated, namely because they added up to more than our original agent came up with, we have been missing out on about £150 A year ?.
I think way too complicated for a first timer, if you get it wrong you could be in bother if they check.
An agent can also best advise on stewardship grants.
Map grid number just refer to normal OS map references, should be easy to find.