Hi 6foot5. No you're right, there is tax leakage although that's offset by the shorter holding time to get the taper relief (I've had mine since June 04). A better way of doing it if you want to hold the shares long-term is with a CFD or spreadbet if you're set up to do that. For me, I wanted the cash for something else and it was clear NLR was going to dip so I sold. If they'd levelled off at above 8 quid I wouldn't have bought them back, but at 650 they were once more extremely good value. My average purchase is about 170 so reducing the gain by 75% is important.
The calculation bit is actually quite easy to do on a spreadsheet so that's not a problem.