One of my dash lights comes on when I rev the engine up (not ignition or oil)< so I assume it has something to do with the generator, can also see big sparks inside it when you rev it up. Does this mean I need to change the brushes, and if so how hard is this to do? Thanks Steve.

Hi Steve,
It is not a difficult job for an auto-sparky but probably something a little much for someone without skills in such matters. If the brushes are worn to the extent it is affecting the output, then you can pretty much guarantee the bushes and commutator will need attention too. See if you can find an "old school" auto electrical place near you, they should know these old Lucas generators backwards.

Regards
Al

Also, check the bronze bush in the end plate as these are prone to wear

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1967 Cooper S - new front bearings to do.
1965 Cooper S shell - Slow progress. No time or money!
1966 Deluxe- next rustoration!
Mk 2 & XJ6 Jags. Less said the better.