Rod,
Four margins and good embossing are the first two things to look for. This stamp appears to have both but margins can be added. These were done one at a time on the sheet and can overlap. This stamp appears aged at the top and maybe just a little dirty over the crown and hair of the embossing.
The standard SG catalogue number is SG57 (2018 cat value £1500 but you can get them for way less than this) but Part 1 doesn't list specimens for this issue. The QV specialised catalogue number is H2s.
The die number can be found in the vicinity of the arrow:

Pennyblackie,
The SG QV specialised states that stamps from die 1 are known without any trace of the die number and that they are seldom clear anyway. The catalogue value you've quoted is for a die 1. Die 4 is also known with the specimen overprint, cat. value £11000
Hope this helps
