Is that from their website, because mostly that stuff is aimed at people needing copies of recent certificates. Why not give them a ring, or pop in next time you're at the Berks Record Office (it's next door).
Yes, I normally put down the details in a letter; the forms aren't usually designed to accommodate FH requests. You can include anything that you think might be helpful: local offices don't take things as literally as the GRO. For instance, if you say your chap was a carpenter, they don't dismiss someone described as a joiner.