As a Trainee Driver it's about £24,000. Slightly above by a few hundred I think.
Thereabouts anyway. 2015/16 pay deal has not been done yet but pay anniversary is April so by the time you start it should've gone up a bit.
Once qualified it jumps to £47,133 currently.
Good luck!
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Actually I'll correct myself as I've just seen an old advert, and applying the difference in the full drivers rate since then, gives a trainee rate of £23,932, which sounds about right.
If you're on the GN side you'll get one Sunday on overtime, or a Saturday rest day work, to do part of your training for the Moorgate tunnels, so this will tip you into £24,000.