Thanks for the replies, yes it is a loose estimate.

Wide range of U figures for brick/air/brick cavity...
- 1.70 (Gov)
- 1.37 (CBA calculator)
- 1.37 (microgen calculator)
- 1.00 (1980)

UWE give a breakdown re typical figures...
- 1.80 (early brick/air/brick cavity wall)
- 1.70 (1972 brick/air/100-dense-block cavity wall)
- 1.00 (1980 brick/air/100-LIGHT-block cavity wall)
- 0.60 (1985 brick/air/125-LIGHT-block cavity wall)
- 0.45 (1990 brick/ins/125-LIGHT-block cavity wall)


I am interested in 1949-51 brick/air/brick, brick returns around windows & a few as ties, hygroscopic black mortar, cavity dirty re snots & narrow (more 35 than 50), bridged wall ties, well ventilated, porous bricks.

UWE data...
- 0.06 - Outside wall surface
- 0.12 - Brick
- 0.18 - Air Cavity
- 0.12 - Brick
- 0.08 - Gypsum Plaster
- 0.12 - Inside wall surface
- Total R = 0.68
- Total U = 1.47

BuildDesk data...
- 0.0600 - Outside wall surface
- 0.1325 - Brick outer leaf & Mortar
- 0.0000 - WELL VENTILATED AIR LAYER
- 0.1821 - Brick inner leaf & Mortar
- 0.0722 - Gypsum Plaster (600kg/m^3)
- 0.1300 - Inside wall surface
- Total R = 0.58
- Total U = 1.73

BuildDesk data...
- 0.0600 - Outside wall surface
- 0.1325 - Brick outer leaf & Mortar
- 0.1800 - UNVENTILATED AIRSPACE: HORIZONTAL HEAT FLOW
- 0.1821 - Brick inner leaf & Mortar
- 0.0722 - Gypsum Plaster (600kg/m^3)
- 0.1300 - Inside wall surface
- Total R = 0.76
- Total U = 1.32

Cavity is not open, but not unventilated re numerous air bricks.
So for my own use re 1949-51 I think UWE 1.47 is more appropriate?