If a TF supplier doesn't have the flexibility to give you what you want, you want a different supplier. Whilst everyone has their standard packages, I had no problem getting pricing on options to suit my requirements...
metals warp with heat, it's what they do. But a stove that's uncontrollable is obviously getting air from somewhere it shouldn't. Are there any "fixed" apertures with screw adjustment? Is there a thermostat and has it's calibration been checked? Does the door need a new rope?
Sorry, Dave, consumer grade Karcher is not brilliant these days. The plastic pump heads distort, and frost kills them completely. See if you can find a service/ parts book online for that model and you'll get a better idea of what's doable.
I can't fault Turkingtons for us, some of their manufacturing is done at Windows 2000 round the corner here in Ards, and Apeer do a big proportion of the composite doors..