I think Ill do it here as I have covered some of it and some the questions here are related to things I was going to cover and I dont want to someone to spend time moving all the posts around.
Tickets
Take a print out from the manual (it is permissible for TOCs to provide them when it would be helpful to the passenger) or find someone willing to look for it in the manual (under shipping links).
Avoid EMT (and possibly SWT though I am not sure how well the ticket offices are aware of SWTs policy not to sell these) and pop over to FCC at Kings Cross (your experience here may be different.) I suggest Charing Cross may be the best place to but I havent tried there, Victoria in theory should also be good.
Euston may also be worth a try as they are used to selling shipping link tickets to Dublin.
South Eastern ticket offices may be more used to selling the fare (and they price it so might find it difficult to argue that it cant be sold by them.) Try places like Canterbury, Folkestone and Ashford International.
The usual suggestion about trying a large station which may be more used to selling unusual tickets or complicated Itinerarys also applies here.
A control ticket is needed, this gets retained by the check in staff at Calais on your return, I didnt get one and they just photocopied my return rail ticket instead and said that it was ok on this occasion. I suspect if someone were to do this trip regularly and without a control ticket issues may arise eventually. Apprently it is quite common for pepole to turn up with out a control ticket so they are used to it.
Railcard discounts are available; the time restrictions are waved for these tickets. This is a SDR, singles and period returns are not available.
Despite what Southeastern might like to think the tickets are valid on HS1 (the routing is "STENA P&O" not "P&O NOT HS1" and there is nothing in the manual about them not being valid) and the guards seem more than happy about it, in fact 1 guard even commented that he had seen the fares on Avantix so presumably they can also be sold on-board.
Buses
The bus from Dover Priory goes from outside the station every 20 mins costs £2.00, takes 5 mins and concessionary bus passes are accepted, this drops you off outside the front of the ferry terminal, Not you will also get the bus back from here, though there isnt anything to say so and in fact the sign you will see coming out of arrivals points to the bus shelter for national express, this is not the where the bus stops. Having said that I overheard a passenger getting on the bus talking to the bus driver saying that where the bus pick up keeps changing so it may be best to check before leaving arrivals, not after leaving as I found departures was locked up, there were no P&O staff around and I couldnt get back into arrivals as the doors had been locked as soon as the passengers had left. The last bus (according to the driver of it) back to the station is at 20:40
The bus in Calais goes from the shelter outside the terminal and goes to the town centre and the railway station and costs 2.00 flat fare, however I didnt have any chance to get any cash on the way (due to the machine at Dover station not wanting to give my card back) to exchange for Euros and walked to the center in about 15 mins. Come out of the terminal, turn right and you will see a sign directing you up the ramp and round the car park, walk up this, follow it round, then follow the road over the bridge and keep going. The route is signed.
Ferry
The trip takes 90 mins, beer is £3.20 a pint and hot food reasonable for this type of travel i.e. £7.49 for a choice of 7 items for breakfast.
Check in is 45 min before departure and you will need a passport and the relevant visas depending on your passport obvious I know but anyone can search Google.
If you have limited mobility or use a wheel chair then you will need assistance to embark and disembark at Calais due to the steep series of ramps that you have ascend/ descend, coming home I saw a few people needing to stop on the way up to get their breath back.
You will be bussed from the terminal at Dover to immigration where you will need to get off show passport and then get back on the bus. Coming back you will be bussed directly to the terminal to walk through immigration. At Calais you will be bussed to and from the terminal, it seems that you only need to pass through immigration on the way out, both French and British and immediately after having your bags x-rayed; you then wait in a departure lounge (read room with seat and toilets) for the bus.
Foot passengers are only accommodated between 08:10 - 19:35 to Calais and 08:30 22:00 to Dover.
Any question, just ask.
Oh and if people could list which stations they have success in buying this ticket at it might be helpful to others.
Edit: added HS1 info, last port bus and foot passenger times.