I have just found out the side wall to my house is only half a brick thick. I am aware this isn't great for insulation. Is it possible to build up the wall on the outside to make it a solid nine inch wall or cavity wall rather than making loads of mess inside the property?
If i where you i could be very concerned about the structural stability of the house. This wall was obviously built by a gerry builder and is in contravention of current building regulations. The only time i have come across a single skin external wall is on timber and steel frame properties that have steel studs to which the brickwork is tied. Then insulation and plasterboard is fixed to the studs. The brickwork in these cases is not taking the weight of the roof of floors. However in traditional build the whole weight of the roof and floors is taken on the external brickwork. It would be best to get a surveyor out to have a look and recommend remedial work necessary to make the wall more stable and insulated.
Take a careful look at the wall.
Are you writing that the wall is made of bricks laid on their edges?
This was common in the 1930'3 for cheap holiday homes but, only for internal walls. Usually they had timber studs to help them.
Was this an internal wall?
Are there a number of piers supporting the roof?
Which way do the joists run? Parallel to the wall?
The wall probably has the heat retention quality of a single pane of glass, not at all suitable for an external wall.
However, that's what you have and the best way to deal with it is to cover the outside with polycarbonate sheets attached to 2x2 timbers, leaving a two inch gap at the top and bottom.
This will solve the biggest problem, that a damp wall, provides an express route out for any heat you put into the building.
The next thing is to solve the heat loss. This can only be done by sticking polystyrene sheets, tightly butted together and covering the whole wall.
I will say that one inch of polystyrene completely covering a wall will reduce your heat loss through that wall by 90%.
However, a better job is to use two inch thick polystyrene.
There is no point in going above five inches as, at this point you cannot measure the heat saved.
Once in place, wet plaster using Thistle finish.
Put several coats on, working towards half an inch thick, you can leave a thinner coat but, if someone bumps against the wall you will get a dent.
It's worth remembering that if the option of external insulation is viable then it is often preferable. Firstly it doesn't reduce the room sizes and secondly it increases the insulated mass of the house which makes it easier to maintain a steady internal temperature. Of course it could get expensive if you need to start altering guttering, etc. Without someone quoting for your exact project it wouldn't be possible to tell if the extra cost is worth it.
I don't know how easy this would be to do DIY although there are many companies with web presences who would do it for you.
Something to consider.
If you make a habit of turning your heating down or off, then having the insulation on the outside will not do you any favours as the room will be slow to heat up as you have those cold walls tacking all your heat.
If you are retired and are living there all the time and never turn the heat down or off, then the mass is good as it can help carry the daytime warmth from the sun into the evening.
While insulataion on the insidse will make the room smaller, the room will yheat up much more quickly.