Grumps wrote :
So, not having wads of cash bursting their way out of my back pocket, what's
the best way to remove the paint from said door and start again? Paint
stripper, heat gun, sand paper (I do have an angle grinder ?

If it is really in need of stripping, then either of the first two.
BTW, if the door is around 7' x 7' - ish, you can get a brand new
insulated, remote control operated and insulated roller door for less
than £500 - assuming you are willing to self install.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk