I got a plumber to do this job and now I'm very glad I did.

Replacing the ballvalve on a 'fortic' cylinder which had to come out of the cupboard to get access.

Drained the cylinder using the gate valve at the bottom. Disconnected the pipes at the sides. Turned the cylinder so the drain was at the front to tilt it to drain some more.

BVeing inquisitive I looked inside the drain and said "it's awfully mucky in there considering this is a soft water area".

Plumber stuck his screwdriver in and moved the muck. The muck had been blocking the drain and the cylinder was very far from being empty. In fact most of the cylinderful started ****ing out and very fortunately there were two of us and two buckets handy to run a bucket relay between the cylinder and the sink.

I think the "muck" might have been a bit of cylinder insulation that got inside from cutting the tappings.

Owain