Morris Minor Owners Club

Well, new fan belt fitted, but the ign light still plays up.
Most of the time when I start up it's fine, and goes out straight away. On occasion though, it stays bright for around a minute after start up, before going out.
Once it goes out, it's fine. A new cut-out box to be considered?

Assuming that you have a dynamo I would suspect the brushes,
an easy job to replace them and cheap too! They are probably
sticking until a bit of heat builds up.

Thanks for that!
Are there instructions anywhere re: replacing brushes for this relative novice?

Haynes Manual details the brush replacement, but there aren't any pictures though I don't think.

There's details and an exploded drawing of early and late types of dynamo.

JOJAX.....remove the dynamo(straightforward...3 bolts) and
you will see,on the 'dip stick' end that it is held on by two very
long screws. Remove these and the end plate will pull off complete
with the two brushes. If the face of the brushes where they run
on the commutator are blackened and pitted then that will be your
trouble. The only tricky bit is holding the new brushes up out
of the way against their spring pressure when you slide the end plate
back on. All done, another 50,000 miles minimum to go!!

Just following on from Willie's post - if the commutator (the segmented copper bit that the brushes run on) is badly burnt or looks like a ploughed field, it will not be worthwhile renewing the brushes. A replacement dynamo could be the best option then.
I've also had instances in the past with intermittent ignition warning light illumunation when the brushes are not worn but have stuck in their holders by carbon dust. However, if it's dismantled then a set of new brushes is logical.