Not strictly a kit car, but I am rebuilding a Reliant SS1 and the a frame round the window/quarterlight is shot, I need some H section mild steel, dimensions are 21mm x 18mm about 1.5 mm wall thickness, anyone know of steel merchants who would stock anything like that? Preferably near Northampton but that may be a bit optimistic.

I've used Arenastock in Letchworth before but they only supply commercial lenghts linky
If there aren't any local smaller stockists try Berkshire metals in Tilehurst linky
They carry quite a few unusual profiles and will supply shorter lengths - don't know about mail-order though.
HTH.
Cheers, Pewe10