My first time with a Victorian house... and it's damp...

No need to tank wall, give it a few months & wall will dry out.
As to air bricks, using periscope liner will allow air flow & any moisture will soon evaporate.

 

Might be worth replacing the whole downpipe, you can get cast iron effect guttering that will suit the age of the property.

Nothing of huge concern, as Kiab says, the wall will dry out.

The air brick is a must to keep adequate airflow and prevent damage to the timber.

 

Air vents in wall in alley way, ideally one course up from path,stops water running in, use periscope liners, posted linky earlier, will take a few weeks to dry, how long depends how soaked the wall is, warmer weather will help.
Make sure you got around 150mm minimum between joists & ground,to avoid them getting wet, if you got floorboards up, might be a good idea check joiss for rot & spray them using garden sprayer with a preseravtive, Everbuild Lumberjack triple protection is very good (TS again).
Sort drainpipe problem, you can use Stixall (only from TS) available in white, clear, to seal & water proof joint between pipes.

Once wall is dry your good to plaster wall.

http://www.screwfix.com/p/telescopic-underfloor-vent-black-220mm-x-215mm/12025?kpid=12025

http://www.screwfix.com/p/terracotta-air-brick-terracotta-76mm-x-229mm/13873

 

Joint between drainage channel & wall, a bead of Stixall should work ok.

 

Don't let his head get too big!

 

Kiab never argues. When you say 'other forums', was one of them DIYnot?

Seems like everyone is in a argument there...

 

A dilute coat of pva on brickwork, allow to go tacky, now the the debate, I like to use Thistle bonding plaster for undercoat,but I'm old school, & times have moved on, a lot of peeps now prefer to use Thistle Hardwall plaster nowadays, as it's quicker drying & has a high impact resistance,& works well on a lot of surfaces like brick, block,etc, scratch surface to give a good key for topcoat Thistle multi finish plaster.

 

Here a linky: https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0ahUKEwi3xuOKmaLSAhXJBcAKHWIaAP8QFggvMAI&url=http://www.british-gypsum.com/~/media/Files/British-Gypsum/SITE-BOOK/SITE-BOOK-Plaster-Systems.pdf&usg=AFQjCNH4dY5R3tTNVgu4iwMrT0YpoR3j9A

Depending how hard those bricks are, you might need to rake out the joints a bit to give a key for the undercoat.

 

The only problem in using bonding or plaster direct on the wall you could still encounter damp or black coming through due to condensation, I only speak from experience, I tried this before and ended up hacking it back off , rendering in cement with a scratch coat then a coat of bonding to create a nice flat surface for the finish plaster which we have not had any issues with it since, as suggested give wall a through clean pva coat render and scratch level bonding coat final plaster skim, I'm no expert but I tried the other way before and still had issues with black showing through, only difference my wall was stone built, might be different with brick but be prepared to start again with just plastering
Just think about it put a piece of concrete outside with a piece of plaster by the side see which one crumbles first when it rains (this is extreme but just giving an example)

 

Agree cement render scratch coat might be a better option to using hardwall,the trouble is I'm here & not there to have a nosey, bonding is consider old fashion by a lot today & gone out of favour being applied direct onto brick, most likely not be suitable, hence sugesting hardwall which a lot of peeps prefer.

 

You shouldn't use gypsum backing coat, it's hydroscopic, any damp in the wall and you will get a damp patch. Renovating plaster would be my choice, or lime plaster.

 

Correct the wall needs to breathe you can archive this by installing vents in the wall at intervals, as in the photo seen earlier if one side is open this is fine as the wall will breathe, in my case the wall was rendered both sides and the quotes I had for a damp course were extreme and in the end plan not needed as the vents installed outside and inside was enough to allow the moisture to evaporate and no more condensation, lime plaster works but also allows mortar or brick stains to come through
Again I can only comment on my house it's stone built with a mix of stone/brick and mortar, the biggest issue really if you want to be honest is double glazing and central heating which wasn't present many years ago, it's now warm inside and no draughts and cold outside so walls do not breathe and moisture cannot evaporate if you left your windows open all the time you would probably not have an issue but it's not nice