Finally got round to it tonight. Did a double flush but when I was putting water in to flush it I was only getting about 7.5 - 8 litres in out of a capacity of 11.8 so looks like there's still a fair bit in the block.
I know there's a drain plug near the turbo but DaveT has a point, it may have been in there without removal for 18 years, what's the chance of it shearing?