I have to replace a couple of floorboards, I cant get to the joist at the one end as it goes under a wall. Could I put a timber batten and screw it to adjacent boards then use this as a make shift joist?
Another possible 'fix' if say you have to cut the floorboard between 2 joists is to screw a decent batten or some ply underneath the adjacent floorboards. Span it across at least 2 boards either side, hold it up from underneath, then screw from the top.
You are quite correct,we used to use a piece of 2x1,you just had to pilot drill it first otherwise you could split the 2x1,it used to be called a "deadman" back in the early days.