Free and Cheap Insulation Official Discussion

If the house is converted to two flats ie Ground and first floor, then both flats need to sign up for insulation as there must be 2 separate guarantees.
I have been surveying for cavity wall insulation for the last 10 yrs on the south coast, and the industry has really moved on now. There are very few problems now, but its is true that the walls must be filled top to bottom to avoid cold patches.
You certainly dont have to have scaffolding to get round a conservatory
You have the option to have it drilled from inside and in the rooms above on the immediate wall above the conservatory, or you can have it brushed off, where they will fit a cavity barrier like a chimney brush which will hang vertically next to the conservatory allowing you to have the rest of the house insulated. Most walls covered by a conservatory are pretty well protected and indeed insulated by the conservatory any way. The heat loss from the uninsulated area above a brushed off conservatory would be very minimal.

There are companies that will cavity insulate extensions only but usually charge around £200 for the first 30 square meters and then 4 or 5 pound per square metre after that.
Shame you are not on the south coast as I know one or two but south Worcs is not on the area.
Try yell.com and ring everyone on the list
Dysons or Crump and Sons may cover Worcs

Bristish Gas seem to be overtly strict when surveying. As long as you have a dpc you should be able to have cavity insulation. The fitters can knock out an air brick to expose the dpc.Thousands of houses are rendered down to the ground, often to protect aginst driving rain in coastal towns so I dont recommend if you are on a cliff top, but if on a normal housing estate in town then fine,I suggest you try your local installer direct.
If your gas fire has a kw rating over 7 kw then you need to have a core vent fitted for co safety reasons before insulation can be installed an the same applies for wood burners over 5 kw and also open fires if the opening is either over 600mm wide or in a very small room, probably like a mid terrace from the 1900s.
These days the rules on a core vent are not as strict as they were when I first started.

Cavity wall Limitations: The Government-sponsored and Local government promoted schemes only use companies who use rockwool insulation. This material will not work in stone-built walls as the rockwool catches and won't flow evenly through the cavity.

If you are turned down for free cavity wall insulation on these grounds, find a company that uses pre-expanded polystyrene beads, which will flow. We used Global Insulation of Sheffield. Phone: 01909 550850.

I have a 1934 detached house and it was insulated in 2003 by Miller Pattisons using Rockwool. They did a very good job and we have never had any problems. They did forget to fill some holes back in, but sent a team back swiftly after a phone call.
So i can recommend them if you are in Salisbury Wilts or the surrounding area.
I think they are a national firm so may cover the areas that some of the forum members reside.