I will try and do a photo step by step for the brickwork, but for now, here's what I can remember happening..
1) Start with a white undercoat
2) Do an 70/30 mix of VMC Green Ochre/water over all of the brickwork so that it flows nicely over the whole thing without getting lumpy. It should also give you some initial colour variations and leave to dry.
3) Do a 60/40 mix of VMC Green Ochre and VMC Deck Tan and drybrush over the whole thing.
4) Do a 30/70 mix of VMC Green Ochre and VMC Deck Tan and drybrush over the whole thing.
5) Do a wash of GW Gryphonne Sepia into the shadowed areas, especially between the bricks and under overhangs.
6) Do another wash of Gryphonne Sepia over some of the bricks to change the overall colour of some bricks. You can also do this with a black wash for some really different bricks.
7) Do a very light drybrush of white over the whole lot to finish the brickwork.
To get the really worn, stained look there are two final steps, but these involve more advanced weathering materials.
To get the green mould stains on the bricks paint some streaks and spots of the AK Interactive Slimy Grime Light weathering liquid. With a damp brush blend the green areas into the brickwork a bit so it's not quite so vibrant.
9) For the muddy, dirty look around the bottom, gently brush on some dark weathering powder, like Forgeworld Dark earth. You don't want too much, just enough to get stuck in the paint's texture.
For the rusty gate I gave it a black undercoat and then dotted on some of the Model Mates Rust Effect. It's probably one of the best weathering liquids I've come across as it created a nice rusty patina without anything else needing to be done.
The roof was an interesting experiment.
1) Start with a patchy undercoat of VMC Field Blue over a white undercoat
2) Do a watery (60/40) mix of VMC Leather Brown/Water over the top of this to darken it up a bit and give the bluey/grey a brown tint and shadows.
3) Water down some GW Temple Guard Blue (60/40 - paint/water) and start painting thin stripes down the roof as if the water hits the top and runs all the way down. You don't need to be too neat about this as any runoff or spashes onto the brickwork below the roof just looks like dirty runoff
4) Keep building up the layers of this green, making sure it's streaky and patchy.
5) Do some small stipples of neat Temple Guard Blue in some areas for a really bright patina
6) A final wash of Devlan Mud into the shadows and around the edges gives you a bit of depth and pop.
I hope those were useful. I'll try to get some pictures next time around