Hi Phil,
From your post, lots of woody growth, lots of borers and then fungal attack, pruning may help,but you are prolonging the agony of the Lemon. The costs and time factor, saw and dig out the butt if possible, drench the area with a good fungicide, back fill. If posible replace with a new improved version but a few meters from the old one, to try to stop any soil born nasties transfering to the new specimen. We all like to hang on to old faithful things, but there comes a time to say goodbye.Good luck, Peter r