Best to drain but not necessarily all the way. If you don’t drain the coolant can leak into the inlet port and needs removing from the cylinder. Theoretically could lock/damage the engine if cranked over, but qty that would leak would be minimal. I always like to remove plugs and crank engine over before attempting to start just in case any coolant residue entered cylinder.
Check inlet manifold face for warping, in particular the brass coloured bushes can pull out over the years and lead to reduced clamping force on the gasket which will never seal no matter how tight you make the nuts. Here’s my story and thoughts on the plastic manifold. Of course the ultimate solution is to get hold of a VVC 160 alloy inlet manifold and support brackets and the problem will disappear if it is the issue.
https://www.the75andztclub.co.uk/for...5&d=1441785047
https://www.the75andztclub.co.uk/for...6&d=1441785054