Reducing the size of an existing, very deep, door lining

[jayc89]

We have an old 15" door lining, which is probably as old as the house. It's near enough butted against the returning, external, wall so I want to reduce the width of it from 805mm to accommodate a standard 762mm door, and allow me to fit some insulation against that external wall (not much, admittedly, 37.5mm insulated PB might have to do). 

 

Given the existing lining is approx 380mm deep, I'm struggling to find an off the shelf replacement, having some timber cut to length is looking like it will cost approx £200 per stile, and I doubt the existing frame will come off in one piece to allow me to shorten the rail and bring the existing stile in. 

 

Anyone have any smarter ideas than sucking up the crazy cost of new timber?

[ProDave]

Fit a standard door liner and line the rest of the width of the opening with plasterboard round the reveal, like you do on windows.

[Dave Jones]

make a liner either out of 2 normal ones joined together or use a sheet of mdf and make your own to size.