What looked like a weeping seal on the tank neck matured into a pin hole with a jet of water from the top of the tank shoulder

Now don't get me wrong the tank was a late 70's vintage and was due a change but I had hoped to do it on my terms. The like for like swap wasn't because the foam insulation is now three times as thick and even though the feed pipes were in the correct place the tank wouldn't push back far enough because its fatter - so some pipework had to be moved to get it in.
The old tank was a bu99er to get down stairs because of the weight so in an idle moment this afternoon I cut the top of - well what a shocker! I keep posting about how hard the water is in our area
but this was a surprise - half the tank full of scale its a wonder I had any hot water at all. It weighed in a 44kg (in a 900 x 450 tank) and I've not knocked it out of the dome yet?