Insulating single brick extension

[ScoobyDoo]

Purchased a new property a year ago and steadily getting through the refurb.
All of downstairs is now done apart from the utility room which is a single brick extension with a seperate toilet area.

Whats an economic way to insulate the single brick walls?. The room will be heated with a gas central heating radiator. Only one wall in the utility is single birck, approx 12ft long and one wall in the toilet only 5ft wide.

I was thinking.

1) Cover walls in damp proof membrane
2) Batton
3) Put polystyrene sheet insulation between battons , something like Celotex Insulation Board
4) Plasterboard
5) Skim

I dont want to loose too much from the width of the room and dont want a costly job, one that i can do myself.

 

[ScoobyDoo]

Thanks for all the replies, so best option is to DPM wall, batten, insulation, board and skim?
Or forget the DPM all together? I've ripped the old board off the wall and they did DPM, batten and board, no insulation apart from a bit of rockwool.