My sympathy: I'm still using a Made in England B&D D720 which I must
have bought around 1975 so I feel your loss.
Mine's something of a Trigger's drill having had many brush
replacements and I even replaced the main bearings once, after
over-enthusiastic use with a plunge router attachment made them too
I think I may have replaced the switch once. I definitely replaced the
switch lock button after that disintegrated and flew away everywhere.
I turned a new one from a scrap of an old plastic toothbrush handle
(using the drill in the bench stand and switching it on and off from
the wall IIRC) and I used an old biro spring to replace the missing
A quick look at Google suggests that it is still possible to buy the
same type of switch so maybe it isn't quite time for your 400H to go
to the great workshop in the sky?