Hi Daniel,
I'm going to presume that you've applied for a London or Greater London based position. On that basis your training salary would start at £20,600.
Your training consists of 2-4 weeks of classroom training, usually at the Horsham training school and then followed by 10 days of shadowing a mentor. That isn't literally 10 days in a row but rather 10 days that your mentor is rostered to work based upon thier personal shift pattern. Here you will be taught how to dispatch different train services (they vary depending on the train operating companie(s) that operate from your particular station and the dispatch plan specific to that station). For example, the type of train dispatch would differ between Southern, South Eastern and GWR services).
Once you have completed your mentoring and your mentor believes you are ready then you are assessed by a Competency Development Manager. Provided you pass and are deemed competent you are then allowed to dispatch individually. This is when your training salary increases to the full salary which is £23,600. Every Sunday that you work, whether it is rostered or overtime is always additional pay and you'll usually be rostered for 2 out of 4 Sunday's per month.
There is normally reasonable overtime available so you can easily push above £2000 per month if you want.
You are also potentially joining at a time when our salaries are due to increase by a minimum of 5% plus another future increase within the next few months or so.
The railway is a good place to work and once you get the hang of dispatching it's not a difficult role unless you struggle with working outside.
There is good cultural diversity and it's not hard to progress provided you are willing to put effort into your applications moving forward. You can apply for internal vacancies after 6 months and trainee driver after 12 months.
GTR isn't a bad company to work for.
Hope this helps, drop me a line if you need more advice/help.