Hi drunk_scotsman,
The £23k is before tax. The £1k p/m is yours to do what you want with - there is no automatic transfer to HSBC.
It's v. hard to guess what allowances etc. will amount to but I've heard a figure of £3-4k p/a being banded about for duty pay as being a reasonable guesstimate. This is, again, before tax and the tax arrangements with duty pay are slightly different to those with your normal salary.
If you sit down and do the numbers you'll find that the first-year take-home pay will be the EQUIVALENT of a gross salary of £44k (ish) p/a.
DISCLAMER: Please remember that all of the above is based on the deal for last year's scheme - it may not necessarily be the same this time around!
V1R