Noco

Clearing part of the 'far' basement in preparation for removing some old pipes that get in the way of using the space efficiently.  This is the area where the ancient and rusting boiler is located. 

Apparently..."Ideal Britannia, Sectional Domestic, Neo-Classic and XLB Domestic Boilers are provided with Noco doors, designed to pre¬heat the secondary air supply, air-cool the baffle plates and give ample access for stoking and cleaning. The heated secondary air is of considerable value when the fuel is smaller than normal or of a reactive nature, effecting an appreciable increase of efficiency."

Looks like a door to me.