GWR is paid 4weekly in arrears, with the pay cycle running a week ahead of the payday.
We were paid yesterday, 26th May. So next payday is 23rd June, then 21st July and so on.
The pay cycle runs to the Saturday before the Friday. So this means that the on 26th May, I was paid for the period 30th April to 20th May. June will be 21st May to 17th June and so on.
So when you start you'll either be paid 1 week or 4 weeks in your first pay, depending where in the pay period you join. If you join on Monday, and payday is Friday, you'll effectively work 5 weeks before being paid, but you'll get a full 4weeks pay as you'll be working all of the pay period you join. Conversely, join the week before payday you'll get a week, join the week after and you'll only get 3weeks.
Hope this helps!